I need to subtract a constant

I have a question about doing subtraction in Forms. I need to subtract a constant from another field but return a zero if the first field is blank. This is for a mileage form, and not all fields are filled in. I only want to subtract a constant if there is a value in the "Miles Traveled" field. I have tried:

event.value = getField("Miles Trav 1").value - 30.0;

This works except when the "Miles Trav" field has a value, but if there is no value, then it returns a minus number. I need an IF THEN where it only subtracts if there is a value entered. I'm a real novice at this and I'm stuck.