I migrated to gmusicbrowser in 2017. Before that, I used Songbird, since about 2013. From 2013 to 2017, I have playlists indicating what month a song was added. However, gmusicbrowser shows them all as being added in January 2017.

My question is whether I can trick gmusicbrowser into thinking these songs were added before they actually were. This would make it easier for me to use smart playlists and weighted random.

There is no easy (via gui) way to do it, but you can either edit the gmbrc file with a script, each song is one line, fields are tab separated, dates are in unix time (seconds since epoch). Otherwise I can give you a command-line that sets the date for a song, still needs a script though.
If you don't know how to make the script I can help you, just give me an example of your playlist that contains the dates.

Quentin, thanks for the quick reply. Sorry, I did not get a notification that you replied. Do you want me to save a playlist, then go into gmbrc file and find it, then paste it the way it's represented in that file?

No just send me one of the "playlists indicating what month a song was added". I'm not quite sure what you mean by that, does the file contains a date for each song, of is it the playlist itself that is named after the date? I need to know how to get the date from the playlist.

Quentin, thanks so much for the reply. I was able to fudge the date for a single track:

1083 2013 05-6 2013 Stereolab > Krautrock\x00Prog Stereolab-esque 1367448011 Ege Bamyasi Can Can 320 0 2 0 0 Can - One More Night.mp3 mp3 mpeg-1 l3 Glam, Prog & Fusion 1616115742 1595452599 336 0 0 1484131490 /storage/0000-0000/Music/C 1 100 44100 13494092 5 One More Night 3 1972
and it worked exactly as expected. I see how playlists are stored:

list002: !SongArray::Named 8946 8945 8944 8947 8791 8790 8793 8963 8964
So it looks like I could group my songs into playlists, based on the month I want them to show as added, then set all these timestamps to xx/01/20xx (but in timestamp format). It sounds like this is where I would need the script. I would need it to loop through those IDs, find those tracks in the library based on ID, and change the timestamps on them to what I want. How do you recommend I go about this?

So, for instance, let's say I have these playlists:

  "May 2013": !SongArray::Named 9037 9033 9030 9036 9035 9029 9034 9032 8993 8996 8999 8992 9001 8991 8994 9000 8997 8998 8995 9002 9003
  "December 2013": !SongArray::Named 8946 8945 8944 8947 8791 8790 8793 8963 8964

To answer your question, the playlist has the month and year of the added date. The day doesn't matter to me and can just be '1'.