Convert `$time.utc` to YYYY-MM-dd format string in MongoDB aggregation

aggregation

 {
   $concat: [
    { '$substr': [{ '$year': '$time.utc' }, 0, 4] },
    '-',
    {
      '$cond': [
        { '$lte': [{'$month': '$time.utc'}, 9] },
        { '$concat': ['0', { '$substr': [{ '$month': '$time.utc' }, 0, 2] } ] }, 
        { '$substr': [{ '$month': '$time.utc' }, 0, 2] }
      ]
    },
    '-',
    {
      '$cond': [
        { '$lte': [{'$dayOfMonth': '$time.utc'}, 9] },
        { '$concat': ['0', { '$substr': [{ '$dayOfMonth': '$time.utc' }, 0, 2] } ] },
        { '$substr': [{ '$dayOfMonth': '$time.utc' }, 0, 2]}] 
    }]
  }
}