[Edit: this is too verbose, I edited my previous post to give a simple one-paragraph description, read that instead.]
Following up my previous post, here are the steps to reproduce:
- This bug occurred with both Anki version 24.04rc2 = b84fce4 and version 23.12.1 (with Qt6, and using Windows 11).
- On your computer, use a North American time zone that observes Daylight Saving Time.
- In Tools / Preferences / Review, make note of the hour that is set for “Next day starts at”. By default, this is 4:00 am.
- Wait until the current time of day is a few minutes before that next-day changeover hour (e.g., 3:58 am).
- In Stats, look at a deck where the histogram bars in the Reviews section show that you studied normally (i.e., reviewed at least one card) on Sunday, March 10, 2024. That is the date for changing from Standard Time to Daylight Time in 2024, and this change always occurs at 2:00 am.
- (As I write this, it is March 21, 2024 so it should be easy to locate the right histogram bar from 11 days ago in the Reviews section. But if you are trying to reproduce this at a more distant date in the future, it may be helpful to use the Date Calculation feature of the Calculator app in Windows to calculate the exact number of days between the current date and March 10, 2024.)
- In Stats, look at the Calendar section and mouseover to the little square that corresponds to Sunday, March 10, 2024: it should be blue, and the mouseover pop-up will show how many reviews were done on that date (it should be non-zero for the deck you chose). As mentioned above, the Reviews section of Stats will also show a normal non-zero histogram bar for that date.
- Wait until a few minutes after the next-day changeover hour (e.g., 4:02 am).
- Look at Stats again. Now in the Calendar section, the little square that corresponds to Sunday, March 10, 2024 is white, and the mouseover pop-up shows 0 reviews. The Reviews section of Stats still shows a normal non-zero histogram bar for that date.
- You can look at Stats for any other deck where you reviewed at least one card on Sunday, March 10, 2024. The above-mentioned Stats / Calendar anomaly occurs for all decks.
- In the Calendar section of Stats, change the year to 2023. Interestingly, the little squares for Sunday, March 12, 2023 and Sunday, November 5, 2023 (start and end dates for Daylight Saving Time in 2023) do not show the same anomaly as for Sunday, March 10, 2024: they are normal and blue (assuming you reviewed at least one card of the deck on those dates).
- In the Calendar section of Stats, you can change the year back to 2024, or close the Stats window, or exit from Anki and restart if you like.
- At this point I refrained from reviewing any cards. I’m not sure if it was actually necessary to do so in order to reproduce the problem.
- Wait until a bit less than one hour after the next-day changeover hour (e.g., 4:58 am). Look at Stats again: the Calendar anomaly still persists.
- Wait until a bit more than one hour after the next-day changeover hour (e.g., 5:02 am). Look at Stats again: now the Calendar anomaly is gone, and the little square for Sunday, March 10, 2024 is back to normal: it’s blue, and the mouseover pop-up shows a non-zero number of reviews.
PS,
Note that the color change from very dark blue squares to lighter blue squares between the two images is associated with the choice of “last 12 months” versus “all history” checkboxes.