from datetime import date, datetime
 
messages = [
	{"author": {"userid": "123", "nickname": "Name1"}, "content": "Some text1", "createdTime": "2021-01-12T15:41:52Z"},
	{"author": {"userid": "456", "nickname": "Name2"}, "content": "Some text2", "createdTime": "2021-01-11T11:30:21Z"},
	{"author": {"userid": "789", "nickname": "Name3"}, "content": "Some text3", "createdTime": "2021-01-11T10:10:47Z"}
]
 
 
dt = date(2021, 1, 11)
 
res = [message for message in messages if datetime.strptime(message["createdTime"], "%Y-%m-%dT%H:%M:%SZ").date() == dt]
 
print(res)
 
 

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