Rather than deleting the file, let's just keep it. Instead, write to a new and different file with the digits of the changing month and year.
ALIAS -l StatsFile return monthlystats- $+ $asctime(yyyy-mm) $+ .txt
On *:Event: {
write $StatsFile some statistic
}
You will merely accumulate more and more files at a manageable rate.
monthlystats-2018-11.txt
monthlystats-2018-12.txt
monthlystats-2019-01.txtetc.