This is part of the announced changes to the Daily Login reward system that came with the latest update. The Daily Login reward system now works in "sets" of 28-day calendars (7- or 14-day calendars for newer players before reaching the repeating 28-day calendars). Each time you reach the 28th day, you will automatically start the next 28-day calendar, which will offer new and different rewards. These rewards are affected by how long you have been playing, with more and greater rewards available the longer you have been playing. However, now there is no way to see your total number of daily logins since your position has been recalculated to appear within one of these 28-day calendars. So if you see a number like "Day 16," this does not mean that you have been reduced to only 16 days of total logins. Instead, your position has been calculated to be positioned on Day 16 of your new current 28-day calendar.