One of the major differences between 2.3 and 2.4 is dates.
All v2.4 dates follow ISO 8601 formats.
From the ID3 specs:
yyyy-MM-ddTHH:mm:ss (year, "-", month, "-", day, "T", hour (out of 24), ":", minutes, ":", seconds), but the precision may be reduced by removing as many time indicators as wanted. Hence valid timestamps are yyyy, yyyy-MM, yyyy-MM-dd, yyyy-MM-ddTHH, yyyy-MM-ddTHH:mm and yyyy-MM-ddTHH:mm:ss. All time stamps are UTC. For durations, use the slash character as described in 8601, and for multiple non- contiguous dates, use multiple strings, if allowed by the frame definition.
The ISO 8601 ‘W’ delimiter for numeric weeks is NOT supported.
Times that contain a ‘Z’ at the end to signal the time is UTC is supported.
MusicBrainz uses XDOR in v2.3 tags as the full original release date, whereas TORY (v2.3) only represents the release year. Version 2.4 does not use/need this extension since TDOR is available.
When converting to/from v2.3 and v2.4 it is neceswsary to convert date frames. The following is the mappings eyeD3 uses when converting.:
Version 2.3 –> version 2.4
If both TORY and XDOR exist, XDOR is preferred.
Version 2.4 –> version 2.3
A MusicMatch extension of unknown binary format. Frames of this type are parsed as raw Frame objects, therefore the data is not parsed. The frames are preserved and can be deleted and written (as is).
An iTunes extension to signify that a track is part of a compilation. This frame is handled by TextFrame and the data is either a ‘1’ if part of a compilation or ‘0’ (or empty) if not.
These are alternative sort-order strings for album, performer, and title, respectively. They are often added to ID3v2.3 tags while v2.4 does not require them since TSOA, TSOP, and TSOT are native frames.
These frames are preserved but are not written when using v2.3. If the tag is converted to v2.4 then the corresponding native frame is used.
A MusicBrainz extension for the full original release date, since TORY only contains the year of original release. In ID3 v2.4 this frame became TDOR.
Apple extensions for podcasts.