code: def _ensure_timestamp_field(dataset_expr, deltas, checkpoints):
      
      measure = dataset_expr.dshape.measure
      if TS_FIELD_NAME not in measure.names:
          dataset_expr = bz.transform(
              dataset_expr,
              **{TS_FIELD_NAME: dataset_expr[AD_FIELD_NAME]}
          )
          deltas = _ad_as_ts(deltas)
          checkpoints = _ad_as_ts(checkpoints)
      else:
          _check_datetime_field(TS_FIELD_NAME, measure)
  
      return dataset_expr, deltas, checkpoints
docs: Verify that the baseline and deltas expressions have a timestamp field.
  
      If there is not a ``TS_FIELD_NAME`` on either of the expressions, it will
      be copied from the ``AD_FIELD_NAME``. If one is provided, then we will
      verify that it is the correct dshape.
  
      Parameters
      ----------
      dataset_expr : Expr
          The baseline expression.
      deltas : Expr or None
          The deltas expression if any was provided.
      checkpoints : Expr or None
          The checkpoints expression if any was provided.
  
      Returns
      -------
      dataset_expr, deltas : Expr
          The new baseline and deltas expressions to use.