Using subtraction I see that one second in serial time equals
1.15740740740389E-05. I'm supposed to calculate the difference between two
times. Is it OK to use this number as a basis for calculating the times and
the differences between them?

This way milliseconds will play a role even though I don't really need to
display them. (I need to be able to show that 4:17:26.9 is earlier than
4:17:27.0 even though they'll both display as 4:12:27.)

Is this an OK shortcut to take when dealing with times? Are there any side
effects that I need to know about?