how to apply built in functions to formula fields in forms?

I have 2 time fields: etime and stime

I need to calculate number of hours between these times, rounding up, in a formula field.

I tried:

((etime - stime) / 3600000).round();

but it wont apply the round() function when I test, and when I go back to editing the application and viewing the formula, it deletes .round()

I'm not sure how else to do this, can anyone help?