Roger,
Thank you so very much, this is the most amazing solution! As a calculated column it works perfectly. After struggling so long to find a solution I can tell you I feel a bit emotional! 
Three questions if I may:
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I see you say I can change it to a measure. Would that be preferable even if the calculated column is already performing as I hoped?
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If there is no good reason to change it to a measure, ignore this question no 2: I tried to create a new measure by copying the code over as a new measure, but then get the error message: “EARLIER/EARLIEST refers to an earlier row context which doesn’t exist.” Not sure what I could be doing wrong. Do I need to change anything in the code moving from calculated column to a measure?
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Then lastly, your elegant solution provides as asked the time difference in days between the job open dates for when jobs are performed. I posted a different question here where I ask for the difference in mileage to be displayed. Is it possible to use this same solution in this post (for days difference) in my other post for the mileage (Meter_KM) difference? If yes, please help me with the how? I hope this would be possible, as this seems such an elegant solution!
With (you have no idea with how much) great appreciation,
William