How to remove negative timevalues?

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I have a query where I calculate the elapsed time by subtracting Time
Out with Time In (24H time, no date in the timevalues). The problem is
that some of the Time Out-values are not available, thus the
calculation gives me a negative timevalue.

I need to replace the negative values with BLANK so that later
calculations of median time is correct. Any one who can help me with
this?

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