The app uses so called "internal date" which is when a message has been "filed" by the server.
Previously, the app was using the "Date:" header, but... this could be completely off by several hours or even years, depending on how wrong the date/time on the sending side was.
I don't see how the option you propose would be useful:
1 - if the sending-side date/time is all wrong, then you would want to see "internal date/time" anyway (and not "January 1 1980" for example);
2 - if it's correct (e.g. the user knows what time zone he is in), then the Date header and the internal date are usually within a few seconds, or at most, minutes, of each other.