Add decimal hours to time

Times in Excel are factional values of 24 hours. One hour of time is 1/24, and 1 minute of time is 1/(24*60) = 1/1440.

As a result, if you have a decimal value for 6 hours, and a time in A1, you can add 6 hours of time to the value in A1 like this:

=A1+(6/24)

With the TIME function

You can also add time values with the TIME function. To add 15 hours to a time in A1, use:

=A1+TIME(6,0,0)

The TIME function saves you from having to remember the formula for converting decimal hours to an Excel time. However, note that the TIME function will "roll over" back to zero when values exceed 24 hours.

For example, note how the two approaches return different results for 25 hours:

=TIME(25,0,0) = 0.041667 = 1:00 AM same day (1 hr)
=25/24 = 1.041667 = 1:00 AM next day (25 hrs)

The TIME function returns the equivalent of 1 hour, while 25/24 returns the full value.

Subtracting hours from time

You may get an error if you try to subtract hours from a time, when the result is negative, because Excel doesn't allow negative time values.

One way to avoid this problem is to use a formula like this:

=MOD(time-(hours/24),1)

Here MOD function takes care of the negative problem by using the MOD function to "flip" negative values to the required positive value.

Another way to avoid this problem is to start with a time that includes a date value. This lets you subtract very large numbers of hours without any danger of getting a negative result. If you don't want to see the date displayed in the result, just apply a time-only number format.