Preschoolers understand little about seconds, minutes, and hours. But they are beginning to understand the sequence of events of the day, or what happened last weekend or will happen. So if you say to your child, "I will see you after work at 4:00", she does not understand how long or when that is. However if you say, "I will see you after your afternoon nap, snack and your outside playtime", she is provided with a sequence of events that have meaning.
These time concepts are particularly important when your child is adjusting to a new environment, or returning to preschool after the summer. And separation adjustments take much longer than what we as parents want to think.
Leaving a physical item of yours with your child, helps as a concrete reminder that she will see you. Sometimes in a new environment the first few days go great, then, when the 'newness' wears off, she may experience some anxiety.
Your knowledge, patience, and positive attitude are particularly needed during these adjustment 'times'.