Via @atomrab: It would be helpful to have a list of the other periods in a given periodization/period collection on the page for the entry of a new period. This would help the user remember which periods from a large collection had already been entered, avoiding duplicates. It would also help to catch errors or typos in dates, since it would be more obvious that there was a gap or overlap between the existing period spans and the one being entered.
Via @atomrab: It would be helpful to have a list of the other periods in a given periodization/period collection on the page for the entry of a new period. This would help the user remember which periods from a large collection had already been entered, avoiding duplicates. It would also help to catch errors or typos in dates, since it would be more obvious that there was a gap or overlap between the existing period spans and the one being entered.