One of the items that came out of the OHDSI Symposium Themis F2F was what to do with DATETIME columns when the raw data has no time. What do you do for start times when all you have is day data? What do you do for end times when all you have is day data? Here is what was discussed but we would like to open it for comment.
Choose default time of 00:00:00 if time is unknown. If you do have time and receive a time of 00:00:00 just shift 1 second to 00:00:01
Work with @clairblacketer to have posted on the Data Model Conventions page under the CDM Wiki under convensions. https://github.com/OHDSI/CommonDataModel/wiki/Data-Model-Conventions