How would you go about finding out when a digital video was made?
When data is in digital format they can be manipulated in any way you want, so the initial data would not be certain in any way.

Videos on mobile phones is a fairly recent thing, however video over phone is nothing new and the first video capable phones are based on codecs back from the 80's. Only the later years you will see phones with more modern video formats due to the increased processing power of the phone.

Meta data and it's possible date data can be wrong. See if you can get the phone's brand and model from the meta data - then you can establish a more accurate period as phones come and go at a rather fast pace.

Then you can combine these data (codec used) to see if you have some overlap time-wise.