I'm sure there are numerous propagation experts on this site, so hopefully they can chime in too. Hardwood cuttings are usually taken during dormancy, before any bud swelling. In fact most accounts I've read say late Fall to mid winter, though it varies by species.
In my case this year, I was chopping the tree anyways so I had nothing to lose by trying to root the cuttings. They were placed directly in moist, but not wet, potting soil and left outside to the elements (after last frost) in shade or dappled morning sun. It was probably late Feb - late June before it was clear which would make it and which wouldn't, so 4 months. In that time the key thing was not letting the first flush of leaves dry out.
On Amazon if you find "The Reference Manual of Woody Plant Propagation" you can search inside the book without buying it. If you search for "Dawn Redwood" you'll find a section with more specific instructions. I'm not sure if it's considered ok to repost that here, but that book basically says take cuttings in late Dec/early Jan, leave in cold storage for a month, then treat with rooting hormone and place in a medium.
I've seen a few references in the past to allowing hardwood cuttings (and air layers) to dry out and callous a bit before putting in a moist medium. I'm curious if others here know the rationale and usefulness behind that.