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

[download]

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

[download]

The question is what do you want to happen if 60 isn't divisible by $delay?

In reply to Re^3: display stuff based on systemclock by ikegami
in thread display stuff based on systemclock by Wire64

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