Excel中的负时间不应该是负数

I'm trying to subtract 2 timepoints which excel is registering as negative when it shouldn't be. Time 1= 00:40 and Time 2=00:20 . Time 1- Time 2 should equal 00:20 but Excel seems to think this is a negative number. The formula I have is a bit more complex as it involves a Look Up function and I only have this problem on 2 of the lookup values (Other values are 00:45 and 01:00). Any thoughts would be much appreciated!I'm trying to subtract 2 timepoints which excel