Fix get_nearest_continuous: accept scalar targets and missing time column

The docstring says ``targets`` accepts "anything ``pandas.to_datetime``
consumes", which includes a bare string or ``pd.Timestamp``. But
``pd.to_datetime("2024-01-01T00:00:00Z", utc=True)`` returns a scalar
``Timestamp``, and ``pd.DatetimeIndex(scalar)`` raises ``TypeError`` —
so single-value cases crashed despite the documented contract.

Wrap a scalar result in a one-element ``DatetimeIndex`` so any
``pandas.to_datetime``-consumable input works.

Also: when the user passes ``properties`` that excludes ``time``, the
helper used to crash with ``KeyError`` deep inside ``df.assign``. Detect
the missing column up front and raise a ``ValueError`` pointing at the
likely cause.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/nearest.py

@@ -137,7 +137,7 @@ def get_nearest_continuous(
         ... )
     """
     _check_nearest_kwargs(kwargs, on_tie)
-    targets = pd.DatetimeIndex(pd.to_datetime(targets, utc=True))
+    targets = _coerce_targets(targets)
     window_td = pd.Timedelta(window)
 
     if len(targets) == 0:
@@ -151,6 +151,11 @@ def get_nearest_continuous(
         filter_lang="cql-text",
         **kwargs,
     )
+    if "time" not in df.columns:
+        raise ValueError(
+            "get_nearest_continuous requires a 'time' column in the response; "
+            "if a `properties` kwarg was passed, include 'time' in it"
+        )
     if df.empty:
         return _empty_nearest_result(df), md
 
@@ -172,6 +177,19 @@ def get_nearest_continuous(
     return pd.DataFrame(selected).reset_index(drop=True), md
 
 
+def _coerce_targets(targets) -> pd.DatetimeIndex:
+    """Accept anything ``pandas.to_datetime`` consumes, including a single value.
+
+    A bare scalar (string, ``Timestamp``, ``datetime``, …) becomes a
+    one-element ``DatetimeIndex``; an iterable round-trips through
+    ``pd.to_datetime`` directly.
+    """
+    parsed = pd.to_datetime(targets, utc=True)
+    if isinstance(parsed, pd.DatetimeIndex):
+        return parsed
+    return pd.DatetimeIndex([parsed])
+
+
 def _check_nearest_kwargs(kwargs: dict, on_tie: OnTie) -> None:
     """Reject kwargs the helper owns; validate ``on_tie``."""
     for forbidden in ("time", "filter", "filter_lang"):

tests/waterdata_nearest_test.py

@@ -265,3 +265,54 @@ def test_forwards_kwargs_to_get_continuous(patch_get_continuous):
     _, kwargs = patch_get_continuous.call_args
     assert kwargs["statistic_id"] == "00011"
     assert kwargs["approval_status"] == "Approved"
+
+
+def test_accepts_single_string_target(patch_get_continuous):
+    """A bare scalar target must round-trip through pd.to_datetime.
+
+    Regression: previously `pd.DatetimeIndex(pd.to_datetime("...", utc=True))`
+    raised TypeError because pd.to_datetime returns a scalar Timestamp for a
+    single-string input.
+    """
+    patch_get_continuous.return_value = (
+        _fake_df([{"time": "2023-06-15T10:30:00Z", "value": 22.4}]),
+        mock.Mock(),
+    )
+    result, _ = get_nearest_continuous(
+        "2023-06-15T10:30:31Z", monitoring_location_id="USGS-02238500"
+    )
+    assert len(result) == 1
+    assert result["target_time"].iloc[0] == pd.Timestamp(
+        "2023-06-15T10:30:31Z", tz="UTC"
+    )
+
+
+def test_accepts_single_timestamp_target(patch_get_continuous):
+    """A single ``pd.Timestamp`` target also round-trips."""
+    patch_get_continuous.return_value = (
+        _fake_df([{"time": "2023-06-15T10:30:00Z", "value": 22.4}]),
+        mock.Mock(),
+    )
+    target = pd.Timestamp("2023-06-15T10:30:31Z", tz="UTC")
+    result, _ = get_nearest_continuous(target, monitoring_location_id="USGS-02238500")
+    assert len(result) == 1
+
+
+def test_missing_time_column_raises_helpful_error(patch_get_continuous):
+    """If the response has no 'time' column (e.g. user passed `properties`
+    that excluded it), raise ValueError instead of crashing with KeyError.
+    """
+    df_no_time = pd.DataFrame(
+        {
+            "value": [22.4],
+            "monitoring_location_id": ["USGS-02238500"],
+        }
+    )
+    patch_get_continuous.return_value = (df_no_time, mock.Mock())
+
+    with pytest.raises(ValueError, match="'time' column"):
+        get_nearest_continuous(
+            ["2023-06-15T10:30:31Z"],
+            monitoring_location_id="USGS-02238500",
+            properties=["value", "monitoring_location_id"],
+        )