If you know your rough time you find yourself in the results as "unknown" then guesstimate based in how far behind/ahead you were of those who finished with you - usually accurate to 5s and probably still more accurare than Strava.
Oh I see - I've done that, I just thought there might be a special way of getting a time that I didn't know about. I got a plastic barcode card thing for xmas so hopefully no more forgetting for me!
How will you get an official time if you didn't have your barcode? When I've forgotten mine I've not got a time.