This is how AWS CloudWatch stores your logs:

How to fetch the last log given a log group name

aws logs describe-log-streams --log-group-name "/aws/lambda/bus" --max-items 1 --order-by LastEventTime --descending
{
	"logStreams": [
		{
			"logStreamName": "2021/05/17/[144]8f55c06a90df4c84a84c19f25d3df6ff",
			"creationTime": 1621261088376,
			"firstEventTimestamp": 1621261080544,
			"lastEventTimestamp": 1621261081829,
			"lastIngestionTime": 1621261164779,
			"uploadSequenceToken": "49616633598735372912937322305425484023201980440079640002",
			"arn": "arn:aws:logs:ap-southeast-1:407461997746:log-group:/aws/lambda/bus:log-stream:2021/05/17/[144]8f55c06a90df4c84a84c19f25d3df6ff",
			"storedBytes": 0
		}
	],
	"NextToken": "eyJuZXh0VG9rZW4iOiBudWxsLCAiYm90b190cnVuY2F0ZV9hbW91bnQiOiAxfQ=="
}

And now to fetch the raw logs:

aws logs get-log-events --log-group-name "/aws/lambda/bus" --log-stream-name "2021/05/17/[144]8f55c06a90df4c84a84c19f25d3df6ff"

When logs are sent to AWS CloudWatch the timestamp is extracted from the message and an ingestionTime is added.