Sets the date pointed to by
value corresponding to the date of the given field.
Caller is responsible for making sure the field exists and has the correct type.
value will point to a newly-allocated copy of the date which should be freed with
g_date_free when no
longer needed (note: this is inconsistent with e.g. get_string which
doesn't return a copy of the string).
the name of a field
a pointer to a Date to set