Okay I had a look at the id3 tags and can’t find anything that looks like the length. I used id3info (part of id3lib) and then tried TagsRevisited on my Windows box. Neither shows any tags the other misses out and neither shows any length tag. I suppose that confirms there is no such tag!
Then I decided to have a look at the tags in iTunes and found it shows the song’s length down to the number of milliseconds. It then dawned on me that it must have the exact length in there somewhere for the gapless playback. I did a search for ‘gapless’ on the hydrogenaudio forums and found this comment:
enc_delay and enc_padding are two fields from the “LAME header” enabling gapless playback. The LAME header usually follows directly a Xing/Info tag which both are located in the first MP3 frame
Now it all makes sense. Is this what you mean when you say “xing tags”?