For our tree collector map, we have numerous date fields that can be nullable. The field staff is running into issues where they enter a date value for a field and then realize they had picked the wrong date field. They go back to edit the field and there is no way for them to clear out the value entered for the date field.
I'm a missing something or is this an overlooked functionality in collector.
Since Collector checks date inputs for validity, I don't think you can go back to a null value. Your best option may be to enter a date like 1/1/1900 that would represent a null/invalid date. Collector will not allow entering a year before 1900.
Since you are using Collector App to update the data, you must be consuming the map services with feature access from a database at the backend that should be the original source of the edits. You can use the Field calculator to update the dates to NULL.
I hope that the date field that you are trying to revert to NULL is not set as auto-update from Editor tracking.
Calculate Field examples—Help | ArcGIS for Desktop
I hope this helps!