anchor.diff:  data requires special support because time-based intervals are not always a
   fixed length.
   
  +Like the histogram, values are rounded *down* into the closest bucket. For
  +example, if the interval is a calendar day, `2020-01-03T07:00:01Z` is rounded to
  +`2020-01-03T00:00:00Z`. Values are rounded as follows:
  +
  +[source,java]
  +----
  +bucket_key = Math.floor(value / interval) * interval)
  +----
  +
   [[calendar_and_fixed_intervals]]
   ==== Calendar and fixed intervals
   
  
positive.code: ---------------------------------
  // TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
  
  If you specify a `time_zone` of `-01:00`, midnight in that timezone is one hour
  before midnight UTC:
  
  [source,console]
  
negative.code: ===== Calendar intervals
  
  Calendar-aware intervals are configured with the `calendar_interval` parameter.
  You can specify calendar intervals using the unit name, such as `month`, or as a
  single unit quantity, such as `1M`. For example, `day` and `1d` are equivalent.
  Multiple quantities, such as `2d`, are not supported.
  
  The accepted calendar intervals are:
  
  `minute`, `1m` ::
  
  All minutes begin at 00 seconds.
  One minute is the interval between 00 seconds of the first minute and 00
  seconds of the following minute in the specified time zone, compensating for any
  intervening leap seconds, so that the number of minutes and seconds past the
  hour is the same at the start and end.
  
  `hour`, `1h` ::
  
  All hours begin at 00 minutes and 00 seconds.
  One hour (1h) is the interval between 00:00 minutes of the first hour and 00:00
  minutes of the following hour in the specified time zone, compensating for any
  intervening leap seconds, so that the number of minutes and seconds past the hour
  is the same at the start and end.
  
  `day`, `1d` ::
  
  All days begin at the earliest possible time, which is usually 00:00:00
  (midnight).
  One day (1d) is the interval between the start of the day and the start of
  of the following day in the specified time zone, compensating for any intervening
  time changes.
  
  `week`, `1w` ::
  
  One week is the interval between the start day_of_week:hour:minute:second
  and the same day of the week and time of the following week in the specified
  time zone.
  
  `month`, `1M` ::
  
  One month is the interval between the start day of the month and time of
  day and the same day of the month and time of the following month in the specified
  time zone, so that the day of the month and time of day are the same at the start
  and end.
  
  `quarter`, `1q` ::
  
  One quarter is the interval between the start day of the month and
  time of day and the same day of the month and time of day three months later,
  so that the day of the month and time of day are the same at the start and end. +
  
  `year`, `1y` ::
  
  One year is the interval between the start day of the month and time of
  day and the same day of the month and time of day the following year in the
  specified time zone, so that the date and time are the same at the start and end. +
  
  [[calendar_interval_examples]]
  ===== Calendar interval examples
  