First, consider moving this question to the tail of your previous thread. That would maintain the flow of the conversation.

The code ref in question,

(Delta_Days($3, $months{$1}, $2, $today[2], $today[1], $today[0]) > 30
+ )? push @log_lines, $line : keep_line($line);
[download]

is asking

date_diff > 30  ?  move_line : keep_line
[download]

which could be restated as

if(date_diff > 30) {
  move_line;
} else {
  keep_line;
}
[download]

That's what the ?: operator does. Delta_Days returns the number of days between two dates. So the > comparison to 30 fits your original request of finding timestamps more than a month old. [1]

Make sense?

[1] That is if you wish to follow the common convention that a month equals 30 days. If you need to do something like anything less than the current day of month in the previous month you'll need a different test.