All listeners are added to a feed and assigned a "Start Date" at that moment. This date is when you add them as a listener, not when they start listening.
The start date is used in drip feeds to determine which episode is released when. All episodes are measured from the previous, with the first being released on their start date.
The start date is also used in Advanced Date-Based feeds where listeners are restricted to only episodes released after their start date.
Changing Listener Start Dates
You can change a listener's start date by clicking "Edit Listener" then "Edit" next to the start date. Set a new date and time, then click "Save".
To bulk change several listeners' start dates at once, click the checkbox next to the listeners, then "Edit" → "Change Start Date...". Select your date/time and click "Continue".
You can read more at this help doc on bulk editing listeners: https://help.helloaudio.fm/article/95-bulk-editing-listeners