I just got burnt by some dodgy date handling in one of my scripts. I worked out wht the problem is, but I wonder if anyone else would be a stupid as I was.

Basically, I'm getting dates from a web form. The dates are read from three drop-down menus - one each for day, month and year.. The values for day and month are all two digit numbers (with leading zeroes for numbers under ten) and the years are all four digits.

Now this gives people the opportunity to input invalid dates (like 2002-02-30 or 2002-09-31) so I need to validate them. I thought I'd be clever and use timelocal to do this. Here's what I did.

# $d, $m and $y have come from the web form. They will
# be (for example) $d = 09, $m = 10, $y = 2002.
# First we need to convert them to "timelocal-friendly"
# values.
$m--;
$y -= 1900;

# Then use eval to check date is valid
eval "timelocal 0, 0, 0, $d, $m, $y";

# Warn on errors
die "Invalid date\n" if $@;

[download]

All very clever, I thought. But it didn't work. Or rather, it worked in most cases, but there were a small number of valid dates that were marked as invalid. The sample values for $d, $m and $y above give one such example.

Without cheating and looking at the value of $@, can you work out what the problem is?

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

In reply to Broken Date Handling by davorg

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