There are many excuses you can come up with, but here are some suggestions for workout times.
Do it early in the morning
My favorite time to work out is between 5:30 and 6:00 a.m. Plus, I know that if I work out at this time, nothing will get in the way of the workout later in the day. It's a beautiful time of day, not too hot, and there's nothing like showering and going to work knowing that I've put in a great exercise (and it allows me to feel superior and look down my nose at those lazy bums I work with).
Squeeze it in during lunch
OK, you're not a morning person. You're busy. You don't have time to work out. Well, suck it up, buster, and sacrifice your lunch hour to the gods of fitness. Bring your workout clothes, do a quick workout, and be back at work ready to tackle the afternoon.
First thing after work
None of those options work for you? Not a problem, my friend. Make it a daily appointment to work out as soon as you get off work. This will also motivate you to finish your work on time so you can get out on time for your workout. It's great to stop somewhere to do your workout before you even get home, because once you get home you'll probably want to relax on the couch, fall asleep at the kitchen table, or rush to the computer.