Industry norm? if progress is through December you call the your Dec. Update, not Jan correct?

Depending on the level and type of schedule, you may consider the time if the schedule is running with shifts and or shifts are from cut-off day to another day, it may give you a false result.
Some of the stakeholders and readers are keen to see the correct cut-off date and it was always a topic for discussion if not compliance.

For me, I always go for the last working hours of the day (Calendar define used and assigned to Project) to make sure that I have a correct cut-off date and not leaving for something that may be in question.

Its called the December update because you are updating or recording progess that happened DURING the month of DECEMBER. Anything after that is forecasted and is in the future.

Why mess with it just use Jan 01 right?

Agree, it is a fail-safe approach, end of a 24 hours calendar day includes every drop of the day and this includes the last infinitesimal fraction of the last second; your approach makes sure of it no matter which calendars/shifts you use.

Usually scheduling software identifies the start of the 24 hours day as the 0 hour of that day and the end of the day as the 0 hour of next day, the same goes for the last day of the month.  Activities use their own calendar while DD correctly uses 24-7 calendar with no exceptions. Not accepting the software correctly identifies the end of the month DD as the 0 hour of the following month first day is being stubborn, we call it swimming against the current.

Shift works that work across midnight hour are not uncommon, 24-7 is perhaps the more common. 

And I thought from the beginning of times all schedulers figured it out on their first update.