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