# Calculating the difference between two time stamps

For example, if cell ** B3** contains

**9:00:00**and cell

**contains**

*C3***17:30:00**, the following formula returns

**08:30:00**(a difference of eight hours and 30 minutes):

**= C3 - B3**

If the subtraction results in a negative value, however, it becomes a wrong time; Excel displays a
series of hash marks (**#######**) because time without a date has a date series number of 0. A negative
time results in a negative series number, which is not permitted.

If the direction of the time difference doesn't matter, you can use the * ABS* function to
return the absolute value of the difference:

**= ABS (C4 - B4)**

This *negative time* problem often occurs when calculating an elapsed time - for example,
calculating the number of hours worked given a start time and an end time. This presents no problem if
the two times fall on the same day. But if the work shift spans midnight, the result is a negative
time.

For example, you may start work at **08:00 PM**, and end work at **5:00 AM** the next day. Using the
* ABS* function (to calculate the absolute value) isn't an option in this case because it
returns the wrong result (15 hours). The following formula, however, does work:

**= IF (C4 < B4, C4 + 1, C4) - B4**

In fact, another formula (even simple) can do the job:

**= MOD (C4 - B4, 1)**

See also this tip in French: Calculer la différence entre deux temps.