<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _ensure_timestamp_field(dataset_expr, deltas, checkpoints):
 """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. """

    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