$ 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

##</code><code>##

$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strftime "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:40

$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strftime "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:40

$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strftime "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:45

$ perl -MPOSIX -le'@lt = localtime; $lt[0] -= $lt[0] % 5; print strftime "%Y-%m-%dT%H:%M:%S", @lt'
2009-07-09T12:14:45