This is not really a bug - more of a small feature request.

The HeaderClock is, well, a clock. It displays the time. It would be nice if it could display the date as well. I spent more time than I will admit trying to figure out why, when I set time_format on Header() to "%c" the time is displayed as 'Mon Jan 1 22:36:22 1900'. In fact, it is because line 92 in header.py says:

return Text(datetime.now().time().strftime(self.time_format))

When a datetime is converted by time(), all date info is stripped.

The time() seems unnecessary. I propose that the line be replaced by

return Text(datetime.now().strftime(self.time_format))

That would allow date formatting strings to be used as well, for those of us who can't remember what day it is. You might also need to change the default width on HeaderClockSpace to auto.

I'll do a PR if you really want one :)