Changes have been made in Git (master):

commit a8fad336b1552a8d85078cd2361cd46cab9ad190
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Nov 14 11:45:49 2012 -0500

     Bug: 11693 Some clients require the duedate/startdate fields.

     This makes no sense to me. The protocol docs state the 
POOMTASKS_DUEDATE and
     POOMTASKS_STARTDATE fields are for the "local" time of the 
duedate. While the
     *_UTCSTARTDATE and *_UTC_DUEDATE fields are for the UTC time.  We have
     no way of knowing, on the server, what the local timezone of the 
device is, and
     the device, likewise, has no way of knowing what the server's timezone is.

     Additionally, the docs also state the ALL date/time data are to 
be transmitted
     in UTC. Some devices transmit both fields to the server, 
formatted as UTC time,
     but with the actual time offset by the UTC offset for the devices 
timezone!

     Until someone can provide a better explanation to me, we will 
treat both fields
     as equal.

  nag/lib/Task.php |    6 ++++--
  1 files changed, 4 insertions(+), 2 deletions(-)

http://git.horde.org/horde-git/-/commit/a8fad336b1552a8d85078cd2361cd46cab9ad190

Actually, I can only reproduce it from server -> client. Adding a new 
task works fine when you add a due/delay date on it.

Works fine on iOS and the TouchDown Android client, but I can also 
reproduce it not working on WinMo6.5.

Tasks with dates set for due / delay are synchronized without dates in 
ActiveSync.

If you set a date in nag, the task is being synchronized to the mobile 
device, but no date is available there. If you set a date on the 
mobile device, the task is re-synchronized but lacks the dates in nag.

Tested with Windows Mobile 6.5 (don't laugh) and current Horde 
packages from pear.