- I am going to cook. I need to set a (or "the"?) timer for half an hour as I started roasting.
- Can I say "I need to time the roast for half an hour"?
- How do you usually say to express that idiomatically?
'To time' as a verb is usually used to suggest measuring a time, not setting one.
You might say:
I'm going to time myself cutting these carrots to see how quickly I can do it!
So, saying 'time the roast' conjures the image of you with a stopwatch waiting to find out how long it takes the roast to finish cooking!
Instead, you would say something similar to the sentence in your first point:
I need to set the timer for half an hour when I put the roast on.