For some reason, this forum's applet failed to translate your Chinese. According to Google Translate, you wrote: ``Can someone please help me to answer, what is the calculation formula of the DURATION function? not covered in the documentation``.
According to the DURATION support page, ``The DURATION function [...] returns the Macauley duration``.
I am not familiar with that formula or Excel function. But according to Investopedia at https://www.investopedia.com/terms/m/macaulayduration.asp , the formula is:
Caveat: I do not take either Investopedia or (especially) Excel documentation for granted. I would test that formula against various DURATION results with well-chosen parameters.
Also beware that if they differ, the mistake is more likely to be in the Excel implementation, in my experience.
But that is just my arbitrary bias against "MSFT" implementations. Again, I have no experience with the DURATION function.
Note: DURATION and most bond functions were originally contributed by third-parties as part of the Analysis TookPak. MSFT never reviewed them for correctness, or so it seems, even after MSFT "mainstreamed" them.