If you are receiving this error “Invalid date/time value. A date/time field contains invalid data. Please check the value and try again.”. This is because the assigning value for the AfterProperties DateTime field in the ItemAdding or ItemUpdating event in the Event Receiver was not formatted correctly.

To resolve this issue you need to format the DateTime value to an ISO8601 string format:

properties.AfterProperties["StartDate"] = DateTime.Now.ToString("yyyy-MM-ddTHH:mm:ssZ");

or by using the SPUtility.CreateISO8601DateTimeFromSystemDateTime method:

properties.AfterProperties["StartDate"] = Microsoft.SharePoint.Utilities.SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now);