Testing out your code, this is what happens. You subtracting the number of seconds from epoc for $t1 from the number of seconds from epoc for $2.

$t1 = 1000
$t2 = 1005
$sec_diff = $t2 - $t1;
print "Time elapsed $sec_diff\n";
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When you call localtime it returning a date time format for the seconds for epoc based upon the time for WHERE YOU ARE!

print "GMT time is " . gmtime($sec_diff);
# Thu Jan  1 00:00:02 1970

print "Local time is " . localtime($sec_diff);
# Thu Jan  1 19:00:02 1970 - for me at least.
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Note: I realized someone else had mentioned the gmtime but I just wanted to explain it further.