In Ulysses, you can display either the creation or modification date of a sheet in the sheet list. Please note that the date display is related to the group's sorting. The modification date will be displayed when your group sorting is set to Manually, By Title, or By Modification Date. When sorted by Creation Date, the creation date is shown. If you work in the iCloud section, the sorting will be synced across all your devices.
It is currently not possible to display the creation date and the modification date at the same time.
Mac
First, go to the menu View › Sheet Preview and check Dates. To change the group's sorting, hover your mouse over the group in the sheet list, click the sorting arrows and select your preferred option.
iPad/iPhone
Open the sheet list, tap the circled ellipsis, select Preview and check Show Dates. To change the group's sorting, open the sheet list menu via the circled ellipsis, tap Sort By and choose your preferred sorting.