Well, Linux is 32 years old; GNU goes back to 1984, and Unix all the way back to 1970! The history of this OS is much older than Linus Torvalds's involvement; he "only" created and maintains the most popular kernel.
But yes, happy birthday to Linux. Many thousands have contributed to making this operating system what it is today and they all have my utmost thanks for it.
I think it's a joke about how UNIX timestamps work. They count milliseconds from January 1st 1970, 00:00:00 UTC, which is 4pm the day before in PST. So the happy coincidence is that they invented UNIX at the very millisecond when its clock starts.