Use difftime() in checktime() as Deuce suggested

Fix some newly introduced MSVC warnings:
  conversion from 'time_t' to 'uint32_t', possible loss of data

Should these local vars be time32_t instead of uint32_t?

I'm not clear why the 2 step conversions now (?):
	time_t -> uint32_t
	uint32_t -> time32_t