I am unable to edit the Start and Expiry Dates in User Subscriptions.

Whenever I update the dates, and hit Update or Save, the fields get filled with zeroes and database doesn't get updated.

Thanks.  I agree, using
   $ctime  = $_REQUEST['ctime'];
   $extime = $_REQUEST['extime'];
instead of
   $ctime  = mosGetParam( $_REQUEST, 'ctime');
   $extime = mosGetParam( $_REQUEST, 'extime');
is a better solution.

Single-stepping through the code shows that the mosGetParam() function does a whole bunch of things that are skipped over anyway, so you always get the same result just getting the $ctime and $extime values directly from the $_REQUEST array. 

The same result with better performance.