Convert datetime to UTC beggining of the day - Oracle db : For example:

I've got datetime in Oracle database like: 18/08/21 13:51:23,420460500 (y/m/d) and I want convert to type: 18/08/20 22:00:00,000000000.

Could you please let me know how can I do this?

I've tried SYS_EXTRACT_UTC("MyDate") but it does not work in that case.