Desktop only. The Import tab and all of its sub-pages are part of Librarian Desktop and are not available in Librarian Mobile. To bring new audio files into your library, use Librarian on your station Mac.
TuneStacker needs to know how long each song is in order to build well-timed program logs. Librarian now measures every song's length automatically, as the song enters your library: when you import it, when Librarian finds a new file in your Station folder, and when Touchups By Folder adds it. The duration goes straight into the library's Duration field, so there is no separate Song Lengths step to run any more.
Earlier versions of Librarian had a Song Lengths sub-tab with a Go! button. It's gone, because the measurement now happens on its own. For a check that actually plays your files all the way through, use the Baddie Detector.
Duration values are stored in seconds. A song that's 3 minutes and 42 seconds long gets a Duration value of 222.00. TuneStacker reads these values when calculating how many songs fit in a given hour and whether the log will run long or short.
If a file can't be read when it comes in, because it's missing, corrupted, or in a format macOS can't handle, it gets no duration. A song that's too small to be real audio is flagged as a bad file.
You can find any songs that still don't have a duration by using the filter in the middle pane. Set the field dropdown to Duration and type unset in the filter field. Any songs that came up empty will appear in the list.
Songs with no duration are usually missing their audio file, or the file is in a format that couldn't be read. Check the file path in the Attributes tab and make sure the file actually exists at that location.
Duration values are measurements of the actual audio file, not the EOM or Cue markers you set in the Transitions tab. A song's Duration represents its total length from start to finish; EOM is where AutoCast begins the transition to the next song. Both values matter, and they're stored separately.