Adding Duration Fields

I am trying to add duration fields but if one is blank the total doesn't show.  Any suggestions?

[End Time 1] - [Start Time 1]  =  [Time 1 Total]
[End Time 2] - [Start Time 2]  =  [Time 2 Total]
[End Time 3] - [Start Time 3]  =  [Time 3 Total]

So if [Time 1 Total] is 1 hour and [Time 2 Total] is 30 min and [Time 3 Total] is blank it won't give me a [Total Time] ([Time 1 Total]+[Time 2 Total]+[Time 3 Total])

How do I get a total if one of the [Time Total Fields] is blank (null)

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Mark Comish
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