Difference between time

[madmax88]

Hi,

How is it possible to substract 2 time: Duration (hh:mm:ss) is variable and the target is always 4 hours:

Duration	Target	Difference	Impact
18:04:19	04:00:00	-14:04:19	-1
01:32:20	04:00:00	02:27:40	0

Any idea?