In date conversion to Unix time month values should be decreased by 1 (months are counted starting at zero)

[chsull]

Hi -- thanks for putting this code together! In my testing I found that the Unix dates produced were advanced by one month over their actual time. It look like this is due to the GregorianCalendar constructor expecting months to be zero-based (0 is January). This can be corrected by subtracting 1 from the month value in the constructor call. Thanks!