PID files have a "fun" gotcha in daemons started as part of system boot — two instances of the same daemon may well get the same PID!
This does not prevent using a PID file, after all, you know that this instance of the program has not written the PID file yet, so if you read it and find that it matches $$, you know the PID has been recycled across reboots.
Perhaps the best option would be to place the PID file on a RAM filesystem that starts out empty every time the system boots. This way, there is no possibility of a stale PID file from the previous session and you avoid needless flash writes as a bonus.
Even if the daemon is run from cron, this issue could bite our questioner — the daemon reboots the system if the time is not set within some grace period, so a network failure could cause the system to start, fail to set the clock, reboot, start the daemon with the same PID again, fail to set the clock, ...
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