The significance of the date was that the system used a week counter up to 1,023 weeks, and the default date went back 19.6 years – as with the Telstra issue.
This is a very well documented thing that anyone running a time server should be well aware of.
https://en.wikipedia.org/wiki/GPS_week_number_rollover
What's strange is that the last rollover was in 2019. How did it go this long before failing?
In 2006, the government changed the daylight saving start date to work better during the Commonwealth Games. Telstra misses the memo, and the clock on every phone in Victoria (and possibly NSW) was out by an hour. Their solution? Wait a week unti DST lined up with their unpatched system.
Telstra must really, really hate preventative maintenance work.