mathematical calculation tracker field
I decided to try a different approach - using 'seconds' of UNIX time.
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(round(div(div(date(sub futuredate (date)))60)60)1)
In the Tracker it displays correctly (in hours)
In PluginLIST it's best not to use the 'date/time' formatting but a simple 'number':
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{FORMAT(name="Time to Date")}{display name="tracker_field_futuredate" default=" " format=number }{FORMAT}
This is for a 'simple' use case, as it will also calculate hours in reverse (which, may or may not be useful).