to write a future date on (a document), as on a cheque to prevent it being paid until then
2.
to assign a date to (an event, period, etc) that is later than its previously assigned date of occurrence
3.
to be or occur at a later date than
Word Origin and History for postdate
v.
also post-date, 1620s, from post- + date (v.1) "to assign a date to, to mark a date on." Related: Postdated; postdating. Intransitive meaning "be of an earlier date" is from 1909.