On _my_ systemwill give them without other info andwill tell you how long you've been up since the last reboot.

As I cleanup the last database (/var/log/lastlog) every month, last will only give me the info from this month, but your lastlog may go back further.

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Very cool! Thank you! That definitely looks like the answer to the OP's question!
Mine goes back to Tue Sep 19 16:24:49 2017, which is when I brought this server on-line.

While 'last' does show me the date-time of the most recent boot, and since I can use that date-time to search any log for entries around that time, I am looking for a log file entry that is definitive "system boot starts here".

Thank you for your replies,
~~~ 0;-Dan

What you see from the last command is the ‘definitive "system boot starts here".’

There isn't one because just after system boot logging is still disabled and the root fs mounted read-only.
As every distro has different startup handling the first entry AFTER the reboot is the one that came in just after system logging was enabled, the messages between the end of dmesg (which are in kernel memory, not disk) and that point never are logged (onto disk files).
So the first entry is distribution, kernel and startup method dependent.
Even the message in lastlog isn't WRITTEN on disk until the root fs has been remounted read-write. so after the file system check etc was done before that.
So by looking in the logs to lines after the date/time reported by last reboot you can determine what on YOUR system is the first logged entry.

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the first message in /var/log/messages (this is my debian) is:
but as it was mentioned it depends on the distro and probably even the file /var/log/messages is missing.