The reason is:
Time is POSIX-standard time, i.e. number of seconds since January 1, 1970. Although it contains concrete date, it does not support dates before 1970 and after Jan 18, 2038.
On other hand
DateTime can easily represent 4,712 BCE as well as 21,000 years in the future. So it should be preferred if you don't want your application to break in edge cases.