In the latest update 6.0.1.7+ outsystems improved web service reference, which now allows for nulliable datetime. (See below)
However it does not allow for nullable datetime for web service providers, that means
nulldate in a SOAP web service response will return
1900-01-01T00:00:00. rather than an empty element.
"XML Schema: Structures introduces a mechanism for signaling that an element should be accepted as ·valid· when it has no content despite a content type which does not require or even necessarily allow empty content. An element may be ·valid· without content if it has the attribute xsi:nil with the value true. An element so labeled must be empty, but can carry attributes if permitted by the corresponding complex type."
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Service Studio 6.0.1.7 (2011-11-19)
? What's new in 6.0.1.7
o Web Reference enhancements:
? Support for Null and Nillable parameters and attributes (#253956)
This can be seen in the image below
REF: http://www.outsystems.com/help/servicestudio/6.0/Designing_Actions/Input_Parameter_Properties.htm