Calendar math looks trivial and is full of traps — fence-post errors, leap-year rules with exceptions to their exceptions, months that aren't a fixed length, and time zones drawn by politics. These six tools handle the traps for you. The age calculator gives exact years, months and days (and covers leap-day birthdays and how different cultures count age); the days between dates tool counts calendar or working days with inclusive/exclusive counting explained; the date calculator adds or subtracts days, weeks, months and years — including the infamous "31 January + 1 month" edge case.

The day of week calculator answers "what day was any date" for all of history (and teaches Conway's Doomsday method so you can do it mentally); the countdown calculator runs live to any event with working-days mode; and the time zone converter handles 32 zones, daylight-saving seams and the date line's zigzags.

All date arithmetic uses the proleptic Gregorian calendar — the same convention as virtually all modern software — with the historical caveats noted on each page where they matter.