WEBDAV - User defined fields - Not updating

I am trying to create a contact using WEBDAV, implemented in C#. In my
Contact form, two user defined columns are there.

1. Alumnus - Number
2. CheckDate - DateType.

Alumnus is getting updated properly. But CheckDate is not getting
updated. It is updated in the exchange store ( visible in Exchange Explorer)
properly but in the outlook form it is displaying as 'None'. The following
XML is used for creating a contact:

================================================== ===
?xml version="1.0"?
dropertyupdate xmlns:d="DAV:"d:setdrop
aabutlookmessageclass
xmlns:aab="http://schemas.microsoft.com/exchange/"IPM.Contact.BCG Marketing
Contact/aabutlookmessageclass
aac:contentclass xmlns:aac="DAV:"IPM.Contact.BCG Marketing
Contact/aac:contentclass
Alumnus1/Alumnus
CheckDate1974-01-01T08:00:00Z/CheckDate
/drop/d:set/dropertyupdate
================================================== ===

How to specify user defined date field when creating contact using
WEBDAV ?