Each AVL is timestamped with the date and time of when the current position is read from the GPS. When the AVL is send to the back-end, the server calculates the Age of Reading by subtracting this AVL time from the current time.
At the End-of-Trip, the GPS receiver is switched OFF and the last position (AVL) is therefore timestamped with the data/time of the Trip End. When a new trip starts (before the GPS has lock), this last AVL information will be send with the Start-of-Trip message.
For example: If a trip ends at 3 PM and a new trip starts at 8 PM, the Age of Reading will be calculated by the server as 5 hours (8 PM - 3 PM).
To ensure that the GPS takes fresh readings out of trip, you must configure the unit to surface every x minutes/hours. If the unit never surfaces, the GPS will not be switched on before the start of the next trip and it means that your Age of Reading of the first AVL will basically be the same as the amount of time that the vehicle was standing.
If you, for example configure the unit to surface every 30 minutes and stay awake for 5 minutes, it means that your Age of Reading should be maximum of 35 minutes. If you configure the unit to surface every 15 minutes and stay awake for 5 minutes, the Age of Reading should be a maximum of 20 minutes.
NOTE: In order for the unit to send a new AVL it must acquire 3D GPS lock first. If the vehicle is, for example, parked in a basement and it could not get lock during the time that it surfaced (5 minutes in the example above) it will send the previous AVL and the Age of Reading will be longer than the surface period.