It's possible that the RPi's firmware was updated and along with that the starting datetime was altered.
"and this was useful because it was easy to see how long the system had been on"
Really...? The RPi you bought somewhere between 2014 and 2016, that has an initial datetime of January 1970, will tell you its proper uptime? It's really beyond me why such an inconclusive statistic would be useful.
Why not just check the system uptime with the "uptime" command?
It's possible that the RPi's firmware was updated and along with that the starting datetime was altered."and this was useful because it was easy to see how long the system had been on"
Really...? The RPi you bought somewhere between 2014 and 2016, that has an initial datetime of January 1970, will tell you its proper uptime? It's really beyond me why such an inconclusive statistic would be useful.
The 1970 isn't relevant, but starting at 12am is convenient as it means my wife can check how long the kids have been watching videos just by looking at the top right of the screen.
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Why not just check the system uptime with the "uptime" command?
Convenience really - if a movie is playing, then moving the mouse pops up (amongst other things) the time and pressing escape hides it. This takes all of 2 seconds and doesnt disturb the kids watching and is within the technical ability of my computer illiterate wife. If the time on system startup is always 12am and she sees it is past 3am then she knows its time for the kids to be doing something else 
