very weird time stamp problem

Since a few days I have a hug time-stamp problem, and I’ve got no clue as to why it occurs and what I can do about it.

What happens is this:

I set a post to be posted in the future. Let’s say at 3.00pm.
at 2.55 pm it says that it will be posted in 5 minutes”.

at 3.00 pm though, no new post appears.

at 3.05 it say again that it will be posted in 5 minutes.

and at 3.10 it say that it will be posted in 10 minutes.

so it is counting backwards, until I post it manually.

Since I sometimes go away for 3 months and more but I want to hav a daily blog, the time stamp feature is essential for me.