Adding time onto a variable

I have a date and time field for when an enquiry was received. 

In a calculated column I want to do;

if the enquiry time is after 6pm

then set the date/time as the following day at 9:00am 

Otherwise, give me the date and time the enquiry was received. 

I can do the obvious if statement parts, but I don't know how to set it to 9:00am the following day, rather than adding 1 day for example. 

Thanks