I think that you'll get in trouble with LB's formula with single digit AM hours, instead use:
if len(totext({table.field},0,"")) = 6 then
time(val(left(totext({table.RT_TIME},0,""),2)),val(mid(totext({table.RT_TIME},0,""),3,2)),val(mid(totext({table.RT_TIME},0,""),5,2)))
else
if len(totext({table.field},0,"")) = 5 then
time(val(left(totext({table.RT_TIME},0,""),1)),val(mid(totext({table.RT_TIME},0,""),2,2)),val(mid(totext({table.RT_TIME},0,""),4,2)))
-k