Instructors who are planning ahead are scheduling the releases of posts (such as in the Announcements forum overview). Unfortunately, it is displaying to the students the underlying creation/modification date and not the date that it became available to the students.
There's quite a few MDL issues out there about similar things, but I could not find one that specifically referred to this behavior.
MDL-51852(fixed this issue in emails) https://github.com/moodle/moodle/commit/a6fc917168788eddca7a534b959a5e1b61d205f3#diff-986d7c30e04332151dbde82052cfd2e0R454
- MDL-55424 (open issue affecting the Latest Announcements block)
MDL-63892(solved a regression caused by MDL-35788, but may also have caused this issue with timed/scheduled posts)