Reflection is what you need here:
[{
"/type/reflect/any_value": [{
"type": "/type/datetime",
"value": "1955-02-24",
"link": {
"source": {
"id": null
},
"master_property": null
}
}]
}]
A couple of notes on that: the MQL manual I've linked to is somewhat bitrotted in its details but is still the best documentation that exists on MQL. Secondly, there's what I'm pretty sure is in MQL bug if you use "*": null
or more specifically "target_value": null
in the link
clause above which makes it ignore the outer value you specified... so don't do that :-)