To log your fasting hours during the day, you'll need to first enable time tracking by going to Settings > Settings Tab > Track Meal Time. Once there, toggle the setting to On.
Once enabled, a time button appears next to the green Add Today button.
When you log foods, be sure to set the time when you ate the food accurately.
Charts of your fasting hours and eating windows will begin to populate. Both charts are available at My Nutrition & Wellness > Fasting.
To set a meal time for an entire meal, go to My Daily Log, select the More Options (⋮) button next to the meal, and select Set Meal Time.