Vega-Lite tooltip not showing time in UTC

I have a Vega-Lite spec visualizing timeseries data with ISO8601 UTC timestamps. These show correctly on the X-axis labels, but in tooltip the time is displayed in local timezone, not UTC.

Is there a way to set timezone for the tooltip?

"tooltip": [
      {
        "type": "quantitative",
        "field": "data[1]",
        "title": "Value "
      },
      {
        "type": "temporal",
        "field": "data[0]",
        "title": "Datetime ",
        "format": "%Y-%m-%dT%H:%M:%SZ"
      }
    ]

Link to full spec in Vega editor