Well I can't think of an identifier that would do it for you, so basically, you have to do the math. This looks like it does the job:
alias difftime {
var %result = 0
if ($calc($gettok($2,1,58) - $gettok($1,1,58))) {
%result = $calc(%result + ($ifmatch * 3600))
}
if ($calc($gettok($2,2,58) - $gettok($1,2,58))) {
%result = $calc(%result + ($ifmatch * 60))
}
if ($calc($gettok($2,3,58) - $gettok($1,3,58))) {
%result = $calc(%result + $ifmatch)
}
return $duration($abs(%result))
}
//echo -a $difftime(22:30:00,22:31:05)
prints:
1min 5secs