@benwerd explained here and here — essentially <time class="dt-*" datetime="yyyy-mm-ddThh:mm:ss±hhmm">human representation</time> is the preferred format.
Sorry about the messy state of the mf2 docs at the moment, we’re working on it!
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Ben Werdmuller:
@barnabywalters Btw, I don't actually see datetime format guidelines in the mf2 spec. Hoping moving to the time attribute helps.
8m