Yes, habit is about cues. Time is one cue, but it can be a weak one if your ordinary routine varies. If you set your exercise time as 6pm, but your activity at

Yes, habit is about cues. Time is one cue, but it can be a weak one if your ordinary routine varies. If you set your exercise time as 6pm, but your activity at 5:30 is not routine, then the time cue may not be enough to trigger the habit. You'll lose track of time and suddenly realize it's 6:15, at which point (like this comments says), you'll feel like you failed and you'll rationalize not doing it at all. The key is to entrain a new habit with an old one---exercise when you get up, or on your way home from work, or at some other point in the day where you're reliably doing the same thing every day.