So as of today we cannot feed a parameter into Date & Time thing. We will add that at some point when we have the cycles. But what you can do, is make your own timer that uses that parameter. To do that, you'll want to store your random number into a variable. Just pass your random number to a Functions: Variables thing using it's "Set variable to value" action.
Then incorporate a counter flow that works on some time interval. In this case I made a counter that works on 15 minute intervals.. Then you can increment a variable by 15 every 15 minutes. So say you want to turn a light on randomly between 9PM and 11PM. At 9PM, generate a random number (let's say it's 59), and have your counter increment by 15 every 15 minutes. By the time that counter hits 60 it'll be greater than 59 and you'll have another flow that says "When myVariable > randomNumer, THEN Turn on my light.".