How to add hours to a date and time
Enter the Base date and Base time, keep Add selected, and enter Hours. The clock rolls through midnight and the calendar date advances automatically.
Calendar and clock arithmetic
Date & Time Calculator
Use this date and time calculator to add or subtract years, months, weeks, days, hours, minutes, and seconds from one Base date and time.
Private by design. The Base values and offset stay in this browser page. The app does not upload your calculation or maintain a calculation history.
Shift one date and time
Calendar years and months move first with month-end constraining. Weeks, days, hours, minutes, and seconds are applied afterward as exact civil time.
Base calendar
Base clock
Calendar units
Years and months use calendar-relative lengths. Weeks and days are exact civil days.
Clock units
Hours, minutes, and seconds roll through midnight and update the calendar date.
Calendar units come first
Years and Months are combined into one calendar-month move. The original day is kept when valid; otherwise it is constrained to the last valid day of the destination month. Weeks, Days, Hours, Minutes, and Seconds are applied afterward.
January 31 example
January 31, 2020 at 23:30 + 1 month + 2 hours first becomes February 29 at 23:30, then rolls through midnight to March 1 at 01:30.
Leap-year behavior
The Gregorian rule makes 2000 and 2024 leap years but not 1900. Adding 1 Year to February 29, 2024 constrains the result to February 28, 2025.
Midnight and year rollover
Clock units are exact seconds. December 31, 2026 at 23:59:30 plus 90 Seconds becomes January 1, 2027 at 00:01:00.
Timezone-free civil time
The result has no UTC offset or named time zone. One day is exactly 24 hours. The app does not account for daylight-saving transitions, historical zone rules, or leap seconds.
Choose the right Time & Date app
Use Date Calculator for date-only work, Time Calculator for duration or clock-only arithmetic, and Time Duration Calculator for elapsed time between two complete dated moments.
Learn the method
Read How to Add and Subtract Date and Time for operation order, month-end behavior, exact-second rollover, and worked examples.
Range and page memory
Supported values run from 0001-01-01 00:00:00 through 9999-12-31 23:59:59. Inputs and results remain in page memory and are not automatically saved after you leave or refresh.