Select _to and _from date from time picker

I have to select the start and end date of the time range picker which is selected by a user. I have a postgresql query in which I filter for ranges between the selected time frame:

  and  start_time >= '${__from:date:YYYY-MM-DD}'  
  and start_time <= '${__to:date:YYYY-MM-DD}'  

My problem is, that I need to pay attention also on hours and minutes.
In the Grafana Doc I found " Any custom date format that does not include the : character"???!!!
Does this really mean I cant use formats where I have a “:” between hours and minutes or do I have to do something else (escape the characters etc.).
Because I think colons are in any existing date format.