Time range rounding down to multiple minutes. Is it possible somehow?

I am using a range query on a timestamp in order to select some documents:

        {
          "range": {
            "start_time_UTC": {
              "gte": "now-5m/m",
              "lt": "now"
            }
          }
        },

In this example, if the time now is 16:13:45, it will take 5 minutes away (16:08:45), and then round down to the nearest minute (16:08:00). What I want to be able to do is select documents from the start of the last 5 minute period (from 16:05:00), so something like "gte": "now-5m/5m", but we're not allowed to use multiples of a period in the rounding down in this way. Is there another way to achieve this?