This is so weird.  This has me puzzled so I did some more research.  I see that the modulesettings table has the settingvalue as a string, although I guess you could store it as a date string in the table and then use the Convert.ToDate() function to turn it back into a date.  

But here's the weird thing.  I used the reset index date on the forum administration page and it put a good number in the modulesettings table (i.e. 0).   But when the search indexer was scheduled to run (on my portal every 30 minutes) it went back to that wierd value in the modulesettings table (39030,861368206). 

Here's a dump of the modulesettings table for the forum module's LastIndexDate setting when I clicked the reset date on the forum administration general settings page (good date):

Here's a dump of the modulesettings table for the forum module's LastIndexDate setting after I manually run the re-index content from the search admin page (Good date):

Here's a dump of my table after the search index job runs (Bad date.. not a good number).  It's replacing the period with a comma:

Hi Crispy

I've the same error in a website....
Is it possible to look at this post?
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/107/threadid/198900/scope/posts/Default.aspx

I believe I have corrected this in our development base, which should hit the tracker soon (4.4.4).