In older desktop versions of Anki, there were no letters. 1 day is 1440 minutes. Though learning steps support seconds; maybe AnkiDroid also expects seconds?
If you use FSRS, it is better to leave that step to FSRS.
The Desktop documentation says “if the interval crosses a day boundary, it is automatically converted to days.”
So, I will try to put a long delay (in minutes) for the last step so that it crosses day boundary.
I’m using the older version of AnkiDroid because the new versions (2.16.x) had some difficulties with migrating my collection (especially media) to internal storage. AnkiDroid 2.15.6 does accept steps like 1.5 (probably meaning 90seconds) but not letters; though seconds make no sense to me.
Hope the future version of AnkiDroid 2.17.x can easily work on internal storage at least with regard to the “collection.media”.
Sorry about the issues with migrating storage! That’s a Google Play restriction.
We publish a ‘full’ build of AnkiDroid on F-Droid which allows us to ask for MANAGE_EXTERNAL_STORAGE. This allows your collection to remain in /AnkiDroid and no storage migration needs to take place.
Alternately, if you don’t mind manually updating your app, our Parallel.A series on GitHub includes the 2.17 alpha with FSRS: Releases · ankidroid/Anki-Android · GitHub and can be run alongside a ‘stable’ AnkiDroid release