For efficiency (if it matters), you could express it as NOT leap year if and only if
(year mod 4 != 0) or ((year mod 100 == 0) and (year mod 400 != 0))
This way, 3 years out of 4, it only executes the first test due to shortcutting. In most of the remaining cases, only the first and second tests execute, again due to shortcutting.
