In T66503 was introduced possibility of importing dates.

Now, we have article Foo and its item Q123

harvest_template -template:Foo date P577

• If no date is in Q123, bot wil add it.
• If there is some date, bot will not add it.

harvest_template -template:Foo date P577 -exists:pt

• if there is other date in Q123 than in Foo, bot will add second.

THis works well for items and strings, but for dates there is another factor - precision.
When in one item is date = 1999 and in second date = 1999-09-09, bot will import even less precise value - this is acceptable when date is sourced, but import from wiki is not reliable source.

some parameter like -exists:better is needed:

• article 1999, item 1999-09-09 - not import
• article 1999-09-09, item 1999 - import