Support timestamps that don't have a day name

[GuiltyDolphin]

Whilst the spec technically requires that the DATE of a timestamp is in the format YYYY-MM-DD DAYNAME - Org itself accepts timestamps without the day name, e.g., [2021-06-27]. It could be useful to allow this format.

Potentially (and I haven't looked into whether this already exists), we could have a system where you can specify whether the parser should be strict to the spec, or whether it can allow additional formats e.g., for dates.