(recalculate your time to compensate for wind)
The ideal time trial course would be flat and with no wind.
Ideally, one would ride equally hard in both directions and the time going
out would be the same as the time coming back (I'm talking here of an out
and back course). Alas, that rarely happens in real life.
The reason for time trials is not just to judge oneself against others, but also to monitor progress made through the biking season. One should be able to see the speed go up over time. Most of my time trials are done alone, there is no one a minute ahead of me to try to catch or a minute behind to stay in front of. It's just a personal test and form of exercise that is more enjoyable than riding a trainer indoors, and I almost always workout a lot harder in a time trial than on my trainer.
One of the problems I have, though, is judging my progress from day to day or week to week. One day the wind is calm, the next the wind beats the crap out of me. Did I improve? It's hard to say, I could have ridden better but still had a slower speed because of the wind.
Fortunately, I've found a way to compensate somewhat for the wind based on the assumption that if the wind slows me down one way it might help me go faster by an equal amount in the other directions. It's not perfect, but I feel it helps even out my times due to wind (or changes in elevation). I keep track of my time at the turn around point as well as the finish line to the course. From these I can calculate my average speed going out and my average speed coming back and then average the two speeds. I use the average of the two speeds to calculate an adjusted time for the whole course. (This is not the same as getting an average speed for the whole course.)
For example, let's say I rode a 10 mile course in 30 minutes and it was a windy day so that on the way out it took me 18 minutes but I streaked back in 12 minutes because of a tailwind. My average speed for the whole course would be 20 mph. but my speed on the way out would be 16.66 mph and my speed back would be 25 mph. The average the two speeds is 20.833 mph, more than half a mph faster than my speed for the whole trip. Averaging the two speeds always gives a faster speed than the average speed for the whole course (except when the two speeds are equal - the perfect time trial). Using 20.833 mph to calculate a new time I get 28.8 minutes or 28:48. So if the day had been less windy, I might have ridden the course a minute and 12 seconds faster!
It's not perfect, but it's been my experience that it helps level out variations in time from day to day. If I have a strong crosswind, which slows me in both directions, it's not going to compensate at all - it works best with a headwind-tailwind combination. Also it works best in an out and back course (as opposed to a rectangular course). It doesn't compensate for getting tired and dying in that last mile. The correlation is not exact, and there are other variables I'm sure, but it does help me to make more sense of my times.