The 'format' attribute should only be used to give meaning to string, number, or boolean types.

or else this JSON;

{"Start_datetime": "2012-12-31T23:59", "End_datetime": null}

will fail against this schema;

{
"type":"object",
"properties":{
	"Start_datetime": {"type":["string", "null"], "format":"date-time"},
	"End_datetime": {"type": ["string", "null"], "format":"date-time"},
	},
"additionalProperties" : false
}

Best regards,
José Gonçalves