There is no query for the "current day", you always have to provide the start and end date (in UTC) when doing the query. For example, to query the daily volume of a sensor (ID 2808) from 2018-06-06-10-31-00 to 2018-06-13-10-31-00 you can use:
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.