Date can be either YYYY-MM-DD or YYYYMMDD
There is more info on the wiki.
Yet one problem: Users don't necessarily use the datetime for standard usages, but also for various personalized situations, though that's not related to the purpose of this request.
In sitations like this, I always use 20181023T124730 since it contains no characters that could be confusing operating systems. Note that I left out the Z timezone since I usually use local time only, but that is personal.
After a few more thoughts, I think this kind of request on options could be more easily solved by a conf file. Users could define their own formats as they will as long as the item values are acceptable and don’t trigger bugs. Could save a lot of developing time.