You could use the change price button already included when you add the product to the ticket.
So add product enter price on keypad tap product and select change price.
If it is a time based discount your best option is probably implementing this tutorial Super Simple Happy Hour Price List
EDIT - just noticed your time based price 
You could still use the above setup but reduce price to £1 then when you enter number on keypad it will be a multiple of £1 not good for tracking actual stock sold but would work.
What you actually want could be achieved automatically using a script to convert the actual time of order into price - what that script would be I’m not sure but there maybe already be something on the forum if you look deep enough.
Further thinking on this you could implement a pop-up keypad to ask price during the happy hour when product added to ticket.
Maybe using the above tutorial to constrain time and parts of this tutorial ( Rick's Tutorial 9 - Custom/Miscellaneous Item Button )to setup price change.