Forgetting we're dealing with times for a second, that's usually done using by subtracting the remainder of a division by your slot size.
$ perl -e'printf "%-3s %2d\n", "$_:", $_ - ($_ % 5) for 0..19'
0: 0
1: 0
2: 0
3: 0
4: 0
5: 5
6: 5
7: 5
8: 5
9: 5
10: 10
11: 10
12: 10
13: 10
14: 10
15: 15
16: 15
17: 15
18: 15
19: 15
The same trick can be applied to times.
$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strfti
+me "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:40
$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strfti
+me "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:40
$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strfti
+me "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:45
$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strfti
+me "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:45
The question is what do you want to happen if 60 isn't divisible by $delay?
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