Batch File Subtraction

[eDragon]

Hi, I'm trying to get into batch files, and I'm not sure how to simply subtract two numbers. The code is as follows:
echo How old are you?
set /p age=">>"
echo What year is it?
set /p year=">>"
set /a birthyear=%age%-%year%
echo So you were born in %birthyear%?
pause

However, when it shows %birthyear% its like -1985, I wasn't born in -1985! How do I fix this?

 

[ss202sl]

You have to subtract age from year, not year from age. If you subtract 2015 from 30 you get a negative number

 

[eDragon]

You have to subtract age from year, not year from age. If you subtract 2015 from 30 you get a negative number

Thanks man, just simple math, I'm such a dufus.