Hello @Adam_B, I have not implemented a local logger with Particle devices but I may give you some advices:
Learn how to write properly data into your SD cards.
Once you know how to write data, you will have to store not only your data but the custom timestamp to be sent later. Your routine should ask constantly the actual timestamp.
If you are going to use our library, to send a custom timestamp call the add() method with this structure: add("var-name", value, NULL, timestamp); (please refer to the examples for more information).
Hope this gives you additional hints to make your project!.