From the scant data it would seem that you are using API version 1.1 (currently deprecated).
My wild guess would be that your code is not setting the ResultField flags correctly.
In spite of the name ResultFieldMask.All does not have all the fields, only the most default ones (title, description, url), and DateTime is not one of them. It is one of the rough edges of the legacy API.
Be sure that in the SourceRequest object for web (or news) you set it properly, for example to have the default fields plus
Code Snippet
newsSource.ResultFields = ResultFieldMask.Source Or ResultFieldMask.DateTime Or ResultFieldMask.All
[Example in VB.net for a change]
If this is not the problem, it is hard to diagnose it without seeing the code.
HTH
--Alessandro