Do I understand correctly that the date isn't guaranteed to be of the same format all the time? If that changes, what is guaranteed to be the same always?

I guess since your date format could change, you could use a module such as Date::Parse to tease the date out of any line containing the trigger anchor text. Perhaps you could capture the date like this:

if( $textline =~ m/invoked by \w+ at (.+)$/ ) {
    $datetext = $1;
}
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Then use Date::Parse's str2time() function to morph the date string into a unix time value:

my $datetime = str2time( $datetext ) or die "Unrecognized date format.
+\n";
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