Most likely you're calculating time based on a timestamp rather than a
datestamp, and if you subtract a PM hour from an AM hour you will get a
negative number (for instance 2 AM - 1 PM = -11, or 2 - 13.. On the
other hand if you use a datestamp that includes a fractional portion of
a day and format the cells to show time only, you can subtract
yesterday's PM time from today's AM time and still get a positive
result.

In a blank cell in a sprdsht enter =NOW() then format that cell as a
number including a comma and several decimal points. 6:30 PM or so
yields 38,980.770. You may want to consult Help to read up on serial
dates.