Date and Time Calculations: Leap Years, Business Days and Time Zones

The number of days between two dates depends on whether you count both ends. From 2 November to 13 November 2026 there are 11 days of difference (13 − 2). If you count both the first and the last day, you get 12. Neither answer is wrong; the question is which one the rule you are following uses. Contracts, leave policies and legal deadlines usually spell it out, and different institutions differ. Check the definition before you rely on a count.

A year is a leap year if it is divisible by 4, except that years divisible by 100 are not leap years unless they are also divisible by 400. So 2024 and 2028 are leap years, 2000 was one, but 1900 and 2100 are not. A leap year has 366 days because February has 29. Leap days matter for ages (a person born on 29 February), anniversaries and any span longer than a year; counting by days and counting by calendar years give different results across a leap day.

Adding days is straightforward. Starting on Monday 26 October 2026, adding 90 days lands on Sunday 24 January 2027. Adding months is trickier, because months have different lengths. Adding one month to 31 January has no 31 February to land on, so tools usually clip to the end of the month (28 February in 2027). Different software handles this differently, which is why the same “plus one month” can produce different dates. When a rule must be exact, count days instead.

A business day is conventionally a weekday that is not a public holiday. Counting weekdays is a matter of excluding Saturdays and Sundays; excluding holidays requires a holiday list for the relevant country, region or institution, which most simple calculators do not include. Weekend days also differ: some countries have a Friday–Saturday weekend.

Example: between Monday 2 November and Friday 13 November 2026, excluding the end date, there are 9 weekdays (5 in the first week, 4 in the second). If you include 13 November, the count is 10. Subtract any public holidays that fall on those weekdays. Always state whether the end date is included.

Age in completed years is the year difference, reduced by one if the birthday has not yet occurred this year. Someone born on 15 March 1990 is 36 on 26 October 2026, because the 2026 birthday has already passed. For ages in years, months and days, the month and day components borrow from the previous month, which again depends on month lengths.

ISO 8601 defines weeks that start on Monday, with week 1 being the week that contains the first Thursday of the year (equivalently, the week containing 4 January). This means the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. Monday 26 October 2026 is in ISO week 44. Both 31 December 2026 and 1 January 2027 fall in ISO week 53 of 2026. Using the calendar year with ISO weeks is a common source of reporting errors; ISO provides a separate “week-year” for this reason.

Time zones are offsets from Coordinated Universal Time (UTC), such as UTC+5:30 or UTC−5. Daylight saving time shifts the offset by an hour in some regions for part of the year, on dates that differ by region and may change by law. For scheduling across zones:

• Convert both times to UTC, compare there, then convert back.
• Do not assume a fixed offset for a region across the whole year.
• A day can be 23 or 25 hours long on the days daylight saving starts or ends.
• Specify the time zone in every invitation, deadline and log entry.

Software commonly stores timestamps in UTC and displays them in local time. A Unix timestamp is the number of seconds since 1 January 1970 00:00:00 UTC, and does not depend on the time zone.

Adding hours and minutes requires carrying 60 minutes into an hour. To convert hours and minutes to decimal hours, divide minutes by 60: 7 h 45 min is 7.75 h. This matters for timesheets, where 7.45 is not 7 h 45 min. Check which format a payroll system expects.

• Is the start date counted?
• Is the end date counted?
• Are calendar days or business days used?
• Which holiday calendar applies?
• Which time zone defines the cut-off?
• Does the rule say “within 30 days” or “by the 30th day”?

• Confusing inclusive and exclusive counts.
• Assuming every month has the same number of days.
• Forgetting the century rule for leap years.
• Treating business-day results as holiday-aware.
• Using local time where UTC was required.

The Date Difference, Add / Subtract Days and Business Days calculators do the arithmetic above. Business-day tools skip weekends; adjust manually for public holidays that apply to you. Confirm legal or financial deadlines with the institution that sets them.