Watchdog sends emails with no date

[Atramasis]

On my server (Ubuntu 12.04 with newest Plesk 12) watchdog is monitoring services and when I get an email it has no sending date. In Horde it shows "Unknown Date" in the list and in the header it shows "Thursday, 01. January 1970".
This is a problem, because on my mobile device I set to receive only mails from last month and these watchdog mails seem to be over 40 years old...

How can I fix this?