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Calendar dates & age calculators
Work with Gregorian dates, birthdays, month lengths and date intervals.
Choose the right calculation
Choose whether an interval includes the start or end boundary. Month and year additions use explicitly stated clamping rules when a destination day does not exist. Date-only arithmetic is kept independent of local daylight-saving hour shifts.
Find a tool for your question
Age Calculator
Find completed years of age between a birth date and an as-of date.
Date Difference Calculator
Count calendar days from a start date to an end date.
Add Days to Date Calculator
Find the calendar date a chosen whole number of days after or before another date.
Day of the Week Calculator
Find the weekday name for a Gregorian calendar date.
ISO Week Number Calculator
Find ISO week-year and week number for a chosen date.
Days in Month Calculator
Find the number of days in a Gregorian month, including leap-year February.
Day of Year Calculator
Find the ordinal position of a date within its Gregorian year.
Age in Days Calculator
Count complete calendar days between a birth date and an as-of date.
Weeks Between Dates Calculator
Express a signed calendar-day interval as decimal weeks.
Year Progress Calculator
Find the percentage of calendar dates elapsed at the start of a chosen date.
Anniversary Year Calculator
Find the anniversary calendar year from an original year and a selected anniversary number.
Leap Year Calculator
Check whether a Gregorian calendar year has 366 days.
Unix Timestamp to UTC Calculator
Convert Unix seconds to a UTC date-time string.
Date to Unix Timestamp Calculator
Convert a civil date at UTC midnight into Unix seconds.
Add Months to Date Calculator
Move a civil date by whole calendar months with an explicit end-of-month rule.
Add Years to Date Calculator
Move a Gregorian date by whole years with leap-day clamping.
Calendar Months Between Dates Calculator
Find the signed difference between two Gregorian month labels.
Does adding one month always add the same number of days?
No. Calendar months vary in length. A day-count addition and a calendar-month addition solve different questions.
Useful next steps in other categories
Hours to Minutes Converter
Convert hours to minutes with an explicit conversion factor.
Reading Time Calculator
Estimate reading time from a word count and reading speed.
More date & time topics
Business days & weekends
Count weekdays and apply a holiday list that you supply.
Explore topic ↗3 calculatorsTime durations & decimal hours
Combine durations and translate between decimal hours and hours-minutes notation.
Explore topic ↗1 calculatorsTime-zone conversion
Convert a local date-time between supported named time zones.
Explore topic ↗