Hi @Anonymous,
Do you want to format the Today() into a date and custom time?
Could you please share a bit more about your scenario, where do you want to refer to this custom date and time?
If you want to save a date and time, you should format this as below:
Text(Today() + Time(20,0,0),"mm/dd/yyyy hh:mm:ss")

Note that this only work for a certain custom time, if you want a dynamic time, like a Dropdown list to select, you should add a separate dropdown for hour value.
Add a Dropdown and set the Items as below:
["00","01","02","03","04","05","06","07","08","09","10","11","12","13","14","15","16","17","18","19","20","21","22","23"]
Format the date and time as below:
Text(Today()+Time(Value(HourValue1.Selected.Value), 0, 0),"mm/dd/yyyy hh:mm:ss")

Hope this could help you at some degree.