Days, Weeks & Months Between
Last reviewed: May 2026
Computes spans in multiple units. Handles varying month lengths, leap years, and business vs calendar days. Uses: age, project duration, contracts, pregnancy.1
| Period | Calendar Days | Business Days |
|---|---|---|
| 1 month | 30.44 | 21–22 |
| 1 quarter | 91 | 63–65 |
| 1 year | 365 | 260–262 |
Calculating the difference between two dates seems simple but involves surprising complexity. Our calendar system has months of varying length (28-31 days), leap years every 4 years (except centuries, except every 4th century), time zones, and daylight saving time transitions. The Gregorian calendar, adopted in 1582 and now used globally, averages 365.2425 days per year — remarkably close to the actual solar year of 365.2422 days, drifting only 1 day every 3,236 years.
A year is a leap year if: (1) divisible by 4, AND (2) not divisible by 100, UNLESS (3) also divisible by 400. So 2024 is a leap year (÷4), 1900 was not (÷100 but not ÷400), and 2000 was (÷400). This system adds one day every 4 years to compensate for the ~6 extra hours in a solar year beyond 365 days. Without leap years, the calendar would drift by 24 days per century — eventually putting "June" in what we consider winter.
Age calculation: Your exact age in days reveals your "10,000th day" milestone (around age 27 years, 5 months) — a celebration in some cultures. Pregnancy: Due dates are calculated at 280 days (40 weeks) from the first day of the last menstrual period. Legal deadlines: Statutes of limitations, filing deadlines, and notice periods are counted in calendar days or business days — the distinction matters enormously. Financial: Interest accrual, bond maturity, and option expiration all depend on precise day counts. The finance industry uses several "day count conventions" — 30/360 (treats every month as 30 days), Actual/365, Actual/360 — each producing slightly different results.
A "30-day deadline" can mean calendar days (including weekends and holidays) or business days (excluding them). 30 calendar days from January 1 is January 31. 30 business days from January 1 is approximately February 11 (assuming 2 holidays in that span). Legal contracts should always specify which counting method applies. The difference can be 10-14 days depending on the period, which matters for shipping estimates, payment terms, warranty periods, and regulatory compliance deadlines.
Date differences can depend on location. When it's January 15 in New York (EST), it's already January 16 in Tokyo (JST). A flight departing Los Angeles at 11 PM on January 10 and arriving in Sydney 16 hours later lands on January 12 (crossing the International Date Line). For global businesses, contracts specify "as of [date] in [timezone]" to avoid ambiguity. The International Date Line itself has political jogs — Kiribati shifted the line in 1995 to keep all its islands on the same date, creating the world's earliest timezone (UTC+14).
Calculating dates before 1582 (Gregorian adoption) requires using the Julian calendar, which had a simpler leap year rule (every 4 years, no exceptions). The switch skipped 10 days — October 4, 1582 was followed by October 15. Different countries adopted the Gregorian calendar at different times: Britain in 1752 (skipping 11 days by then), Russia in 1918, and Greece in 1923. Historical date calculations across the transition period are notoriously tricky — George Washington's birthday is February 22 (Gregorian) but was February 11 (Julian) when he was born.
How old are you, exactly? In most Western countries, you turn one year old on your first birthday. In traditional Korean age (until its abolishment in 2023 for legal purposes), you're 1 at birth and age increases every New Year's Day. In some cultures, a baby is considered "one year old" at birth. These differences matter for legal purposes — voting age, driving age, and drinking age are defined by the Western birthday-counting method in most jurisdictions. For precise age: your exact age in days divided by 365.2425 (average Gregorian year length) gives decimal years.
Project timelines require careful date math. The critical distinction: duration (calendar time) versus effort (work time). A 10-day task starting Monday with weekends off takes 14 calendar days to complete. Holiday-adjusted business days vary by country and region — the U.S. has 11 federal holidays, Japan has 16, India has 26+. International project teams must reconcile different holiday calendars. Most project management software (MS Project, Asana, Monday.com) handles this automatically using configurable work calendars, but manual date calculations require awareness of these differences.
The finance industry uses specific day-count conventions that affect interest calculations on bonds, loans, and derivatives. 30/360: Assumes 30-day months and 360-day years — simplifies calculations but doesn't match reality. Actual/365: Uses actual calendar days divided by 365 — common in UK and Australian markets. Actual/360: Uses actual days divided by 360 — common in U.S. money markets. The convention matters: on a $1,000,000 loan at 5%, the difference between Actual/365 and Actual/360 over a 31-day month is $70. Over a year with multiple transactions, these differences compound into meaningful amounts.
Date differences carry legal significance in surprisingly many situations. Statute of limitations periods run from the date of an event (injury, breach of contract, discovery of fraud) and vary by state and claim type: personal injury ranges from 1-6 years, written contracts from 3-15 years, and property damage from 2-6 years. Missing a deadline by even one day can permanently bar a legal claim worth millions. Employment law uses date calculations for FMLA eligibility (12 months of employment, 1,250 hours worked in the past 12 months), COBRA continuation (60 days to elect, 18 months of coverage), and unemployment benefit windows. Financial contracts specify dates precisely: a 30-day money-back guarantee starting January 15 expires February 13 (not February 14), because the start date counts as day one in most commercial contexts but not in some legal jurisdictions where the following day is day one.
The Gregorian calendar used worldwide adds a leap day every four years, except for century years not divisible by 400 — which is why 2000 was a leap year but 1900 was not. This system averages 365.2425 days per year, closely matching the actual solar year of 365.2422 days but drifting one day every 3,236 years. Date calculations spanning centuries must account for these rules: the number of days between January 1, 1900 and January 1, 2000 is 36,524 (not 36,525) because 1900 was not a leap year. Business date calculations add another layer: the "30/360" convention used in bond markets assumes every month has 30 days and every year has 360 days, simplifying interest accrual but creating discrepancies with actual calendar dates. The "actual/365" convention counts real calendar days against a 365-day year. Choosing the wrong day-count convention on a $10 million bond position can create thousands of dollars in pricing error.
Calculating age seems trivial until edge cases arise. In most Western countries, you turn a year older on your birthday. But in South Korea's traditional system (being phased out as of 2023), everyone was one year old at birth and gained a year every January 1 — making someone born on December 31 two years old the next day. Legal age calculations matter for voting, drinking, driving, and contract capacity: in many US states, you reach legal age at the start of your birthday, not at the time of day you were born. Leap day babies (February 29) face unique challenges — in non-leap years, most jurisdictions consider March 1 as their birthday for legal purposes, though some use February 28. Date difference calculators must handle these variations or note which convention they follow.
Construction, software development, and event planning all rely on accurate date arithmetic that accounts for non-working days. A project estimated at 60 business days starting March 3 doesn't finish on May 2 (60 calendar days later) — it finishes around May 23, after excluding weekends and holidays. The discrepancy grows with project length: a 200-business-day project spans roughly 280 calendar days (40 weeks), not 200 calendar days (28.5 weeks). Critical path calculations in project management stack dependent task durations end-to-end: if Task B can't start until Task A finishes, and Task A takes 15 business days and Task B takes 20 business days, the sequence requires 35 business days plus any holidays falling within that window. Failing to convert between business days and calendar days when communicating deadlines to stakeholders is one of the most common causes of missed project deliverables — a June 30 deadline means something very different if the project plan counted business days but the stakeholder assumed calendar days.
Date differences carry legal significance in surprisingly many situations. Statute of limitations periods run from the date of an event (injury, breach of contract, discovery of fraud) and vary by state and claim type: personal injury ranges from 1-6 years, written contracts from 3-15 years, and property damage from 2-6 years. Missing a deadline by even one day can permanently bar a legal claim worth millions. Employment law uses date calculations for FMLA eligibility (12 months of employment, 1,250 hours worked in the past 12 months), COBRA continuation (60 days to elect, 18 months of coverage), and unemployment benefit windows. Financial contracts specify dates precisely: a 30-day money-back guarantee starting January 15 expires February 13 (not February 14), because the start date counts as day one in most commercial contexts but not in some legal jurisdictions where the following day is day one.
The Gregorian calendar used worldwide adds a leap day every four years, except for century years not divisible by 400 — which is why 2000 was a leap year but 1900 was not. This system averages 365.2425 days per year, closely matching the actual solar year of 365.2422 days but drifting one day every 3,236 years. Date calculations spanning centuries must account for these rules: the number of days between January 1, 1900 and January 1, 2000 is 36,524 (not 36,525) because 1900 was not a leap year. Business date calculations add another layer: the "30/360" convention used in bond markets assumes every month has 30 days and every year has 360 days, simplifying interest accrual but creating discrepancies with actual calendar dates. The "actual/365" convention counts real calendar days against a 365-day year. Choosing the wrong day-count convention on a $10 million bond position can create thousands of dollars in pricing error.
Calculating age seems trivial until edge cases arise. In most Western countries, you turn a year older on your birthday. But in South Korea's traditional system (being phased out as of 2023), everyone was one year old at birth and gained a year every January 1 — making someone born on December 31 two years old the next day. Legal age calculations matter for voting, drinking, driving, and contract capacity: in many US states, you reach legal age at the start of your birthday, not at the time of day you were born. Leap day babies (February 29) face unique challenges — in non-leap years, most jurisdictions consider March 1 as their birthday for legal purposes, though some use February 28. Date difference calculators must handle these variations or note which convention they follow.
Construction, software development, and event planning all rely on accurate date arithmetic that accounts for non-working days. A project estimated at 60 business days starting March 3 doesn't finish on May 2 (60 calendar days later) — it finishes around May 23, after excluding weekends and holidays. The discrepancy grows with project length: a 200-business-day project spans roughly 280 calendar days (40 weeks), not 200 calendar days (28.5 weeks). Critical path calculations in project management stack dependent task durations end-to-end: if Task B can't start until Task A finishes, and Task A takes 15 business days and Task B takes 20 business days, the sequence requires 35 business days plus any holidays falling within that window. Failing to convert between business days and calendar days when communicating deadlines to stakeholders is one of the most common causes of missed project deliverables — a June 30 deadline means something very different if the project plan counted business days but the stakeholder assumed calendar days.
Date differences carry legal significance in surprisingly many situations. Statute of limitations periods run from the date of an event (injury, breach of contract, discovery of fraud) and vary by state and claim type: personal injury ranges from 1-6 years, written contracts from 3-15 years, and property damage from 2-6 years. Missing a deadline by even one day can permanently bar a legal claim worth millions. Employment law uses date calculations for FMLA eligibility (12 months of employment, 1,250 hours worked in the past 12 months), COBRA continuation (60 days to elect, 18 months of coverage), and unemployment benefit windows. Financial contracts specify dates precisely: a 30-day money-back guarantee starting January 15 expires February 13 (not February 14), because the start date counts as day one in most commercial contexts but not in some legal jurisdictions where the following day is day one.
The Gregorian calendar used worldwide adds a leap day every four years, except for century years not divisible by 400 — which is why 2000 was a leap year but 1900 was not. This system averages 365.2425 days per year, closely matching the actual solar year of 365.2422 days but drifting one day every 3,236 years. Date calculations spanning centuries must account for these rules: the number of days between January 1, 1900 and January 1, 2000 is 36,524 (not 36,525) because 1900 was not a leap year. Business date calculations add another layer: the "30/360" convention used in bond markets assumes every month has 30 days and every year has 360 days, simplifying interest accrual but creating discrepancies with actual calendar dates. The "actual/365" convention counts real calendar days against a 365-day year. Choosing the wrong day-count convention on a $10 million bond position can create thousands of dollars in pricing error.
Calculating age seems trivial until edge cases arise. In most Western countries, you turn a year older on your birthday. But in South Korea's traditional system (being phased out as of 2023), everyone was one year old at birth and gained a year every January 1 — making someone born on December 31 two years old the next day. Legal age calculations matter for voting, drinking, driving, and contract capacity: in many US states, you reach legal age at the start of your birthday, not at the time of day you were born. Leap day babies (February 29) face unique challenges — in non-leap years, most jurisdictions consider March 1 as their birthday for legal purposes, though some use February 28. Date difference calculators must handle these variations or note which convention they follow.
Construction, software development, and event planning all rely on accurate date arithmetic that accounts for non-working days. A project estimated at 60 business days starting March 3 doesn't finish on May 2 (60 calendar days later) — it finishes around May 23, after excluding weekends and holidays. The discrepancy grows with project length: a 200-business-day project spans roughly 280 calendar days (40 weeks), not 200 calendar days (28.5 weeks). Critical path calculations in project management stack dependent task durations end-to-end: if Task B can't start until Task A finishes, and Task A takes 15 business days and Task B takes 20 business days, the sequence requires 35 business days plus any holidays falling within that window. Failing to convert between business days and calendar days when communicating deadlines to stakeholders is one of the most common causes of missed project deliverables — a June 30 deadline means something very different if the project plan counted business days but the stakeholder assumed calendar days.
Date differences carry legal significance in surprisingly many situations. Statute of limitations periods run from the date of an event (injury, breach of contract, discovery of fraud) and vary by state and claim type: personal injury ranges from 1-6 years, written contracts from 3-15 years, and property damage from 2-6 years. Missing a deadline by even one day can permanently bar a legal claim worth millions. Employment law uses date calculations for FMLA eligibility (12 months of employment, 1,250 hours worked in the past 12 months), COBRA continuation (60 days to elect, 18 months of coverage), and unemployment benefit windows. Financial contracts specify dates precisely: a 30-day money-back guarantee starting January 15 expires February 13 (not February 14), because the start date counts as day one in most commercial contexts but not in some legal jurisdictions where the following day is day one.
The Gregorian calendar used worldwide adds a leap day every four years, except for century years not divisible by 400 — which is why 2000 was a leap year but 1900 was not. This system averages 365.2425 days per year, closely matching the actual solar year of 365.2422 days but drifting one day every 3,236 years. Date calculations spanning centuries must account for these rules: the number of days between January 1, 1900 and January 1, 2000 is 36,524 (not 36,525) because 1900 was not a leap year. Business date calculations add another layer: the "30/360" convention used in bond markets assumes every month has 30 days and every year has 360 days, simplifying interest accrual but creating discrepancies with actual calendar dates. The "actual/365" convention counts real calendar days against a 365-day year. Choosing the wrong day-count convention on a $10 million bond position can create thousands of dollars in pricing error.
Calculating age seems trivial until edge cases arise. In most Western countries, you turn a year older on your birthday. But in South Korea's traditional system (being phased out as of 2023), everyone was one year old at birth and gained a year every January 1 — making someone born on December 31 two years old the next day. Legal age calculations matter for voting, drinking, driving, and contract capacity: in many US states, you reach legal age at the start of your birthday, not at the time of day you were born. Leap day babies (February 29) face unique challenges — in non-leap years, most jurisdictions consider March 1 as their birthday for legal purposes, though some use February 28. Date difference calculators must handle these variations or note which convention they follow.
Construction, software development, and event planning all rely on accurate date arithmetic that accounts for non-working days. A project estimated at 60 business days starting March 3 doesn't finish on May 2 (60 calendar days later) — it finishes around May 23, after excluding weekends and holidays. The discrepancy grows with project length: a 200-business-day project spans roughly 280 calendar days (40 weeks), not 200 calendar days (28.5 weeks). Critical path calculations in project management stack dependent task durations end-to-end: if Task B can't start until Task A finishes, and Task A takes 15 business days and Task B takes 20 business days, the sequence requires 35 business days plus any holidays falling within that window. Failing to convert between business days and calendar days when communicating deadlines to stakeholders is one of the most common causes of missed project deliverables — a June 30 deadline means something very different if the project plan counted business days but the stakeholder assumed calendar days.
Date differences carry legal significance in surprisingly many situations. Statute of limitations periods run from the date of an event (injury, breach of contract, discovery of fraud) and vary by state and claim type: personal injury ranges from 1-6 years, written contracts from 3-15 years, and property damage from 2-6 years. Missing a deadline by even one day can permanently bar a legal claim worth millions. Employment law uses date calculations for FMLA eligibility (12 months of employment, 1,250 hours worked in the past 12 months), COBRA continuation (60 days to elect, 18 months of coverage), and unemployment benefit windows. Financial contracts specify dates precisely: a 30-day money-back guarantee starting January 15 expires February 13 (not February 14), because the start date counts as day one in most commercial contexts but not in some legal jurisdictions where the following day is day one.
The Gregorian calendar used worldwide adds a leap day every four years, except for century years not divisible by 400 — which is why 2000 was a leap year but 1900 was not. This system averages 365.2425 days per year, closely matching the actual solar year of 365.2422 days but drifting one day every 3,236 years. Date calculations spanning centuries must account for these rules: the number of days between January 1, 1900 and January 1, 2000 is 36,524 (not 36,525) because 1900 was not a leap year. Business date calculations add another layer: the "30/360" convention used in bond markets assumes every month has 30 days and every year has 360 days, simplifying interest accrual but creating discrepancies with actual calendar dates. The "actual/365" convention counts real calendar days against a 365-day year. Choosing the wrong day-count convention on a $10 million bond position can create thousands of dollars in pricing error.
Calculating age seems trivial until edge cases arise. In most Western countries, you turn a year older on your birthday. But in South Korea's traditional system (being phased out as of 2023), everyone was one year old at birth and gained a year every January 1 — making someone born on December 31 two years old the next day. Legal age calculations matter for voting, drinking, driving, and contract capacity: in many US states, you reach legal age at the start of your birthday, not at the time of day you were born. Leap day babies (February 29) face unique challenges — in non-leap years, most jurisdictions consider March 1 as their birthday for legal purposes, though some use February 28. Date difference calculators must handle these variations or note which convention they follow.
Construction, software development, and event planning all rely on accurate date arithmetic that accounts for non-working days. A project estimated at 60 business days starting March 3 doesn't finish on May 2 (60 calendar days later) — it finishes around May 23, after excluding weekends and holidays. The discrepancy grows with project length: a 200-business-day project spans roughly 280 calendar days (40 weeks), not 200 calendar days (28.5 weeks). Critical path calculations in project management stack dependent task durations end-to-end: if Task B can't start until Task A finishes, and Task A takes 15 business days and Task B takes 20 business days, the sequence requires 35 business days plus any holidays falling within that window. Failing to convert between business days and calendar days when communicating deadlines to stakeholders is one of the most common causes of missed project deliverables — a June 30 deadline means something very different if the project plan counted business days but the stakeholder assumed calendar days.
→ Use for project planning. Know exact business days to deadline.
→ Subtract holidays manually. Not auto-excluded.
→ 30 days ≠ one month. Months vary 28–31 days.
→ Check business days for contracts. Avoids ambiguity.
See also: Countdown · Age · Pregnancy