Are both dates on each side of the relation using the underlying ISO format timestamp, or is one of the dates typed in manually into the sheet and the other one entered through the app?
I’m thinking you could create a template of the date to lock in the formatting on one or both dates and then use the template for the relation.
Set the Display on the EndDateAgendar user specific column to Date only / short. Then duplicate it with math column (m_EndDateAgendar) and set the Display to Time Only / Without secs. Do a template column to join both (tpl_EndDateAgendar). In sheets, use the below formula on the End Date:
={"End Date V2";ARRAYFORMULA((IF(LEN(T2:T) = 0, "", text(T2:T,"M/DD/YYYY H:MM AM/PM"))))}
End Date V2 should now match with tpl_EndDateAgendar and give you a relation