Actually remove the datetime field from field properties when it is null, instead of leaving it and setting it to null.
Because the jquery datepicker's default localization is English, it could conflict with the UserInterfaceCulture that is used in the StringToDateTimeConverter, when for example the abbreviated name of the month is used as a value.
Merge pull request #184 from Erikvl87/fix-datetimefield-initialize

Fix DateTimeField bugs