Further to Solution 1 it looks like you want to remove the time element of your DateTime. Normally you can just create a new date using the DateTime constructor that takes the elements of the original date that you want to retain. But because you have a nullable DateTime you have to insert an interim step e.g.
DateTime? InstallDate = new DateTime(2022,01,25,13,27,36);
DateTime notNullDate = InstallDate.GetValueOrDefault(DateTime.Now);
DateTime ConvDate = new DateTime(notNullDate.Year, notNullDate.Month, notNullDate.Day, 0,0,0);