I want to be able to add or subtract a specific number of hours from that
time and save it in a different field on the same form.

I tried using the following =DateAdd("h",-1,[fielda]) in the macro under the
set value option but I get an error.

I can get this to work if I create another field called fielda-date and set
the control source to =dateAdd("h",-1,[fielda-date]) then use the macro to
set the value of fielda to fielda-date - but that is the long way.

Is there a way to do this without having to go through the alternate process?

You are using the correct function but it would appear that you are
attempting to use the "field name" as opposed to the "Name of the control"
that has the date you want to use in your calculation.

=DateAdd("h",-1,[NameOfControl])

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HTH
Mr. B
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