Hi BobbyP,
If it is T-SQL statement, why not to use the FORMAT() function?
DECLARE @var DATETIME = GETDATE();
SELECT @var AS [Before]
, FORMAT(DATEADD(DAY,-1,@var),'yyyy-MM-dd 00:00:00.000') AS [After];
Output
+-------------------------+-------------------------+
| Before | After |
+-------------------------+-------------------------+
| 2020-08-17 17:26:01.953 | 2020-08-16 00:00:00.000 |
+-------------------------+-------------------------+