The documentation would seem to imply timestamp is required when submitting multiple datapoints at once using create_all.json where it is optional when submitting just one using datapoints.json.
In fact, value seems to be the only required property when creating just one and all of timestamp, value, and comment are required on create_all.
Sorry, this was my mistake – was using the same url in both calls (datapoints.json rather than datapoints/create_all.json). And looks like the documentation is incorrect, I can create multiple datapoints with daystamp only just fine.