proc in.years s { expr {$s / (60 * 60 * 24 * 7 * 52)}}
 proc in.weeks s { expr {$s / (60 * 60 * 24 * 7)}}
 proc in.days s { expr {$s / (60 * 60 * 24)}}
 proc in.hours s { expr {($s / (60 * 60)) % 24}}
 proc in.minutes s { expr {($s / 60) % 60}}
 proc in.seconds s { expr {$s % 60}}

 set ago [clock scan "2005-12-10"]
 set s [expr {[clock seconds] - $ago}]
 puts "[in.years $s]y [in.weeks $s]w [in.days $s]d [in.hours $s]h [in.minutes $s]m [in.seconds $s]s"