I know a lot of women have posted about this, but seems most post comments are being disabled, SO:
I discovered if you forget to log your period, you can click on it as if you're going to log it today, CHANGE THE END DATE FIRST to the closest date the app allows to when ur actual period ended. Then the rest of that month will become available to use for a start date.
Example, mine started 23rd ended 27th IRL. forgot until today to log. Changed end date to 7/29 as that was earliest available date it'd allow. Then when I clicked on start date the entire mo th of July suddenly became available. Clicked the 23rd and after, updated my end date to 7/27.
