Creating Datetime value for specific time e.g 9am-5pm

Hi is there a way (formula) to create a datetime value but only for specific time range ?.
For example the field needs to generate a date & timestamp (between 01/01/2021 and 31/01/2021) but the time stamps are only for office hours i.e. 09:00:xx and 17:00:xx

Can I use a formula to achieve this or do I need to split out the time and the date and concat ?.
Cheers !