Retrieve the value of a message's Date: header field.
||dkim_get_msgdate() can be called at any time after end-of-headers
has been reached (i.e.
dkim_eoh() has been called).
||A Date: header field was not found, or one was found but could
not be parsed, or the feature was not enabled when the DKIM
library was compiled.
||A UNIX time_t translation of the value of the Date:
header field, returned as a 64-bit quantity.
- Presumes the provided Date: header field is in standard format.