Integer overflow in 'mktime' when checking leap year.

Will there not be an overflow if 'tm_year' is one of [INT_MAX-1899, INT_MAX]
when checking if the 'tm_year' is a leap year.
In glibc they avoid adding 1900 to the 'tm_year' when doing this check in order 
to avoid overflowing 'tm_year'.
I didn't look into details of how 'mktime' is implemented on FreeBSD so I might 
be wrong, but just skimming through, the overflow in 'tm_year' seems obvious?

-- 
Jure