I never have a job finish in the time calculated by the slicer, yet that number is persistent.
Is is always off and sometimes seriously so.
It would be nice to be able to add the actual time back to the gcode file that will display next time the file is used and in the library.
There might also be other notes that might be worth attaching after having run a job.
The time estimates are just estimates. It's never going to be 100% accurate as settings in the firmware, such as acceleration, is very printer dependent. Most people don't even know what their acceleration values are for their printers. So I always take the estimates as a ballpark figure, and I don't think I've experienced any extremely outrageous differences.
Also, I'm not 100% certain that s3d actually tracks the amount of time a print job takes, but if you want to update your gcode files to have actual print time, you can open the gcode file in a plain-text editor (notepad for Windows, TextEdit for Mac, gedit or even terminal text editor [nano] for linux) and one of the last lines contains the print information, including build time. Adjust accordingly, and then in the Machine Control Panel, remove the gcode file from the library, then add it back to the library and it should have the adjusted time.