time counting at -50°C
The best solution I've seen for flexible timing is the DS3231.

This can provide very good accuracy over extended periods with low drift ..BUT.. it is only characterized down to -40degC. 

You could try testing a batch at the extended temperature range, or talk to Maxim about what the absolute limits might be.

A cheaper solution would be the DS3231M which is rated to -45degC operation but is not quite as accurate.
You still need to characterize it at -50degC of course, as it's likely not to quite deliver is usual 5ppm.