If we are talking about "correctness" of numbers, then ... we should measure the "length", that means the "duration of playtime", of a music track in the correct meaning as the amount of used samples per sample frequency.
Mp3tag yet does not provide the number of samples of any music file type.
Mp3tag rounds, I am not shure into what direction, up or down, the playtime to whole seconds.
Rounding up seems to be more logical than rounding down. You do not need more tape when rounding up, but the tape is always too short when rounding down. This is an old practical philosophy from the days of optimal filling a compact cassette.
You can export playtime from Mp3tag to textfile using the system placeholders "%_length%" and "%_length_seconds%", where the latter has the more generic datatype of an integer number, which can be formatted on the target system to any format view you need.
You have discussed above a way to import whole seconds into the Excel application.
It is sort of the application and the datetime datatype it relies on. Keep in mind that floating point numbers sometimes lacks on precision or accuracy. It is safer to transport integer numbers from one application to another application.
If you export from Mp3tag this string ...
"0:0:%_length_seconds%"
... resp. ...
... to an Excel cell format of "Uhrzeit 37:30:55" (german Excel) or user defined "hh:mm:ss", then you will get the right view in the Excel cell automatically.
DD.20100204.1553.CET