I actually never considered the astronomical symbols, that would work too. Not sure I would use emoji though, I mean, at least with Chinese writing it is still writing and once you know how, they're trivial to write, and in the case of year/day/month, about as trivial as any letter of the alphabet.
There's actually a recent precedent for this. The "@" symbol was almost completely unused prior to the late 90's. Email saved it from extinction, but now Twitter and other services have made it a universally understood named-person-entity indicator (which English never had before).
The 2nd and 3rd hanzi/kanji they used were quite literally the words for "moon" and "sun" (at least in Japanese, where they also mean "month" and "day").