You have setup a Visitor Group to run during a certain time period but it runs either early or late.
The first step is to confirm your not running into a known bug as described in the Visitor Group Time of Day Times do not save correctly KB. Once you have confirmed that your not seeing that bug then what you are most likely seeing is a miss-match in Time Zones. Visitor Groups times are displayed and set in server time, not your local time and the time criterion is compared against server time as well. If your server is in say the Eastern Time zone and you are in Pacific Time when you choose to start a group at 2:00 PM, you are setting it to 2:00 PM Eastern Time. To correct this either update your server time zone to the correct zone or manually calculate the time difference and adjust the start/end times appropriately. If you are a DXC customer you can open a ticket with our Infrastructure Support team to have them add an application level setting to change the time zone. This does require a restart of the web app so there will be down time when this change is made in DXC.