How we calculate

DayReckon answers everyday questions about dates and time: how many days until something, what date it will be, how old someone is, how many working days a job will take. Each page gives the answer first and then shows the rule behind it, so you can check it yourself.

Calendar dates, not clock time

Our calculators turn each date into a running day number and do the arithmetic on whole days. That means the answers cannot be thrown off by time zones or by the nights when clocks change for daylight saving. “Today” always means today on your own device, so a reader in Sydney and a reader in Los Angeles each see the right answer for where they are.

Months and years follow the calendar

Adding a month moves to the same day of the next month, and ages are counted in whole birthdays, then months, then days. When a month is too short (adding one month to January 31), we stop at the last day of that month. Birthdays on February 29 are counted on February 28 in common years; we say so on the age page because some countries use March 1 instead.

Holidays and special dates

• US federal holidays follow 5 U.S.C. 6103 and the Office of Personnel Management’s rules for observed days.
• Bank holidays in England and Wales follow the rules published on GOV.UK, including substitute days.
• Western Easter uses the Gregorian computus; Orthodox Easter uses the Julian computus converted to the Gregorian calendar.
• Chinese New Year dates come from published Chinese calendar tables.
• Ramadan and Eid dates follow the Umm al-Qura calendar. Where the month starts on moon sighting, the actual day can differ by one, and the Ramadan page says so.

One-off holidays announced by a government are not included automatically.

How we check our work

The date engine behind every page is covered by automated tests against known results: published holiday lists, Easter tables, ISO week numbers and leap-year edge cases. We re-check the rules and dates on the date shown at the bottom of each page.

Limits

Our results are accurate calendar counts, but contracts, courts and government offices sometimes have their own counting rules, such as excluding the first day or extending a deadline that lands on a holiday. For anything with legal or financial consequences, confirm the rule that applies to you.

Found an error, or want a calculator we do not have yet? Contact us.