I also created similar habit tracker app for mobile https://play.google.com/store/apps/details?id=com.yedev.habi... The unique thing is that I use google drive to sync habits
For the past days, if you click the 3 dots next to the checkbox, there's an option to add checks for previous days using the calendar
For the past days, if you click the 3 dots next to the checkbox, there's an option to add checks for previous days using the calendar