I have a calculation field that has something weird...

If it receives value '-1' (negative one) and gets added 1, the result is 1 (positive one), where it should be zero ( -1 + 1 = 0 ). Lower than minus 1 works fine, just the -1 screws up...

I have to add an IF statement to screen for a -1, for it to set it to zero if 1 is added:

If ( Guest | TA Towel Availability = -1 ; 0 ; Guest | TA Towel Availability + Traffic::Traffic | Towel Count )

Breaking my brains here, too weird for words!

Filemaker 11 Advanced Mac

A simple file showing the problem would be useful. Judging from your code, I would guess that if Guest | TA Towel Availability contains -1, and

Guest | TA Towel Availability + Traffic::Traffic | Towel Count

returns 1, then the field Traffic | Towel Count in the first related record from Traffic contains 2.

Most importantly, it is always useful to know what you would like to achieve.