Just going to comment instead of answer, as I'm not sure if this will work - maybe the "Calculated Number Field" from JIRA Misc Custom Fields plugin? I am horrible at math so I have no idea if you are able to create a formula that would work with dates instead of raw numbers.
Not sure what "trank the transition" means, but the field is calculated dynamically if that makes any difference (will automatically recalculate and update its value when one of the other fields are changed). Sorry my suggestion doesn't fit the bill; hope someone else is able to find you something that works. :)
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