Calendar arithmetic is fiddly — months differ in length and leap years add a day every four. These three tools handle that for you and show the counting method they use.
The age calculator counts complete years first, then whole months, then the remaining days, and also gives your total days lived and the days until your next birthday. The date calculator measures the gap between two dates in days, weeks, months or years, and can add or subtract a number of days from any date. The time calculator adds and subtracts durations in base 60, carrying seconds into minutes and minutes into hours, and converts between hours, minutes and seconds — useful for timesheets, where 7 hours 30 minutes becomes 7.5 decimal hours.