TaskGrid advanced datefilter is not working

The date "at" filters of the the advanced taskgrid filters were not working.

Assumed reason: the date is sent in format Y-m-dTH:i:s to the server, normally Y-m-d H:i:s is used. This was not the real problem since MySQL can deal with the T in datetime values.

Real reason: the dates are stored with the time too, but for filtering 00:00:00 time is assumed, therefore the at filter will never match. This is fixed now.