When subtracting the elapsed time from the 'Approx print time should result in the correct time left. But for example
The approx Print time is 3.5 hours, the elapsed time is 60 minutes which should yield 2.5 hours left to print, but instead it says 14 hours.
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OctoPi 1.8.0 on an Ender 5 Pro.
The calculation you're doing will never be correct, because of the way that the numbers are calculated.
The approximate total print time is made by analysing the file when it is uploaded, and is an estimation and is not guaranteed to be accurate as there's a lot that could influence it.
The print time left number is calculated on the fly based on how much of the print is left, and how much has been printed so far. OctoPrint uses a simpler, faster method of analysing on the fly so it is regularly wrong. If it has a red dot, it *knows" it is not correct. It won't add up because it is not necessarily meant to. As mentioned already, this has been discussed at length already.
The time elapsed is obvious, and is the only 'accurate' figure, since it is literally just a timer counting up.