When track layer is rendered, you can get message closest to specified time (used in track player, hittest tool, etc.). Value of time must be between values for which track layer was rendered. To get such message, use the request:

http://{host}/avl_hittest_time?sid=<text>&unitId=<uint>&layerName=<text>&time=<uint>&
revert=<bool>

{
	"currMsg":{		/* current message */
		...
	},
	"prevMsg":{		/* previous message */
		...
	},
	"index":<uint>,		/* index of current message */
	"layerName":<text>	/* layer name */
}

Formats of current and previous messages are described in the chapter Data format: Message with data.