When possible, dates should be expressed in ISO 8601 and RFC 3339 compatible
Calendar format, that is to say "YYYY-MM-DD", or otherwise a Combined Date
and Time representation (<date>T<time>, so "YYYY-MM-DDThh:mm").
Separators for the date (hyphen) and time (colon) components are required
to conform to both ISO 8601 "extended" format and RFC 3339 required format.
In the case of Date and Time representation, a timezone can be added as per RFC 3339 and the newer revisions of the ISO 8601 standard (which allow for negative offsets):
Z for UTC (e.g. "YYYY-MM-DDThh:mmZ"), or
For more details see examples at Wikipedia page on ISO 8601 and the publicly available RFC at RFC 3339.
Other representations from those specifications are not necessarily supported.
Values of certain variables may be propagated from device reports formally as opaque strings, which happen to convey a date/time value (commonly the device or battery manufacture date, replacement date, last self-test or calibration time stamp, device clock, etc.) in some format, not necessarily a standard one.
While the drivers may convert them from original vendor-provided markup to the standard Time and Date format described above (if the formula is known for certain — e.g. which locale is used by the device, which part of that string is the year/month/day, or how to add offset or prefix for the year, etc.), they are not generally required to do so.