Difference of a field between 2 variable dates

[blaisjoel]

Trying to find the difference from a field when it's the first date of the year and the last date of the year.
I have create an if statement to retreive the first date and last date from a field.
I can't seem to figure how to get the difference of a field where the first and last date applies.
Here is the formula for the first and last date:

Numbervar FirstDate;
numbervar LastDate;


if{@MyDate}={@Day First} then FirstDate:= {vehaudfl.aud_trans_lfmeter}
Else if {@MyDate}={@Day Last} then LastDate:= {vehaudfl.aud_trans_lfmeter}

Now I need to get the difference between the fields value: Last Date-First Date