Unify date formats

We insert dates as '2013-08-28 18:08:48' into the database
and later on query for them like '2013-08-28T18:08:48'.

This breaks for sqlite as it does not have
a designated 'datetime' type. It compares the date
as plain strings, which works for a unified date format only.

Bug 12637

Signed-off-by: Jan Schneider <jan@horde.org>
Use constant and don't rely on correctly set default format (Bug #12637).