Yes, this feature is released and you can use it. In case that you use GroupDocs cloud service when you add date field to the document just switch to the date settings tab as shown on this screenshot
and select format which you needed.
In case that you use GroupDocs.Signature for .NET library you should do next:
in Global.asax.cs where you initialize the component
GroupdocsSignatureSettings class has property “DateFieldAcceptedFormats” this is string array, you can setup there the custom formats and then on prepare you will be able to select one from those formats.