What each tab does
Age takes a date of birth and counts years, months and days up to today, or up to any date you choose in “Age on”. It also shows the age in total months, weeks and days, the weekday you were born on, and how long until the next birthday.
Days between counts the days from one date to another and shows the same gap as years, months and days. The first day is not counted, so Monday to Wednesday is 2 days.
Add or subtract starts from a date and moves it forward or back by days, weeks, months or years. Day of the week tells you which weekday a date falls on, past or future.
How months are counted
Months have different lengths, so the calculator adds whole months first and counts the days left over. From 31 January, one month later is the last day of February, because February has no 31st. A 29 February birthday is celebrated on 28 February in years that have no leap day.
Questions
Is my date of birth saved or sent anywhere? No. The sums run in your browser and nothing is stored, except which tab you last used.
Why does a month-and-day answer differ from another site? Some calculators count a month as 30 days. This one uses the real calendar, including leap years, so the answer matches counting on a wall calendar.
Which time zone is used? Only calendar dates are compared, so time zones and clock changes cannot move an answer by a day. “Today” is the date on your device.