!followage reports a different month count than FossaBot #673

Describe the bug
The !followage command reports a follow duration that differs from FossaBot by one month for the same Twitch user and channel.

To Reproduce

1. Run !followage through Senchabot.
2. Run !followage through FossaBot for the same user and channel.
3. Compare the results: Senchabot's month count differs by one.

Expected behavior
Senchabot should calculate the elapsed follow duration consistently and return the correct calendar month count.

Additional context
This may be caused by converting elapsed days into fixed-length months instead of calculating calendar years, months, and days from the Twitch follow timestamp.