Accept ISO 8601 PT durations and open-ended range NA endpoints

Per copilot review on PR DOI-USGS#247:

- Broaden the duration passthrough regex from `^[Pp]\\d` to `^[Pp]T?\\d`
  so time-only durations like `PT36H` (documented in get_continuous and
  get_daily as valid `time`/`last_modified` values) are preserved
  instead of being parsed as datetimes and silently dropped.
- Per-element NA handling: a `None` / `NaN` / empty endpoint in a 2-value
  range now becomes `..` in the output, matching the docstring contract
  and supporting half-bounded intervals like `[date, None]` and
  `[None, date]`. Previously, a non-string element would raise from
  `.endswith` inside `_parse_datetime`.

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

Author: thodson-usgs

dataretrieval/waterdata/utils.py

@@ -215,30 +215,37 @@ def _format_api_dates(
     if len(datetime_input) > 2:
         raise ValueError("datetime_input should only include 1-2 values")
 
-    # Pass through duration ("P7D") and pre-formatted interval ("a/b") strings
-    # untouched. Anchor the duration check so the bare letter ``P`` / ``p``
-    # appearing inside a normal word doesn't accidentally bypass parsing.
-    if len(datetime_input) == 1:
+    # Pass through duration ("P7D", "PT36H") and pre-formatted interval ("a/b")
+    # strings untouched. Anchor the duration check so the bare letter ``P``
+    # appearing inside a normal word doesn't bypass parsing; allow the optional
+    # ``T`` so time-only durations like ``PT36H`` are recognized.
+    if len(datetime_input) == 1 and isinstance(datetime_input[0], str):
         single = datetime_input[0]
-        if re.match(r"^[Pp]\d", single) or "/" in single:
+        if re.match(r"^[Pp]T?\d", single) or "/" in single:
             return single
 
-    parsed_dates = [_parse_datetime(dt) for dt in datetime_input]
-    if any(dt is None for dt in parsed_dates):
+    # Per-element: NA endpoints become ".." in the output for half-bounded
+    # ranges; otherwise parse. If any non-NA element fails to parse, return
+    # None overall.
+    def _format_one(dt) -> str | None:
+        if pd.isna(dt) or dt == "" or dt is None:
+            return ".."
+        parsed = _parse_datetime(dt)
+        if parsed is None:
+            return None
+        if date:
+            return parsed.strftime("%Y-%m-%d")
+        utc = (
+            parsed
+            if parsed.tzinfo is not None
+            else parsed.replace(tzinfo=local_timezone)
+        ).astimezone(ZoneInfo("UTC"))
+        return utc.strftime("%Y-%m-%dT%H:%M:%SZ")
+
+    formatted = [_format_one(dt) for dt in datetime_input]
+    if any(f is None for f in formatted):
         return None
-
-    if date:
-        return "/".join(dt.strftime("%Y-%m-%d") for dt in parsed_dates)
-
-    # Localize naive datetimes to the runner's local zone before converting
-    # to UTC; tz-aware datetimes are converted directly.
-    utc_dates = [
-        (dt if dt.tzinfo is not None else dt.replace(tzinfo=local_timezone)).astimezone(
-            ZoneInfo("UTC")
-        )
-        for dt in parsed_dates
-    ]
-    return "/".join(dt.strftime("%Y-%m-%dT%H:%M:%SZ") for dt in utc_dates)
+    return "/".join(formatted)
 
 
 def _cql2_param(args: dict[str, Any]) -> str:

tests/waterdata_utils_test.py

@@ -114,6 +114,11 @@ def test_format_api_dates_passthrough_duration():
     assert _format_api_dates("P7D") == "P7D"
 
 
+def test_format_api_dates_passthrough_time_only_duration():
+    """ISO 8601 time-only durations (PT...) are passed through unchanged."""
+    assert _format_api_dates("PT36H") == "PT36H"
+
+
 def test_format_api_dates_word_with_p_is_not_a_duration():
     """Strings containing the letter 'p' must not be misclassified as durations."""
     assert _format_api_dates("Apr") is None
@@ -133,3 +138,9 @@ def test_format_api_dates_date_only_pair():
 def test_format_api_dates_space_separated_still_works():
     """The legacy space-separated format must still parse."""
     assert _format_api_dates("2024-01-01 00:00:00", date=True) == "2024-01-01"
+
+
+def test_format_api_dates_open_ended_range_with_none():
+    """A None / NaN endpoint becomes '..' in the output range."""
+    assert _format_api_dates(["2024-01-01", None], date=True) == "2024-01-01/.."
+    assert _format_api_dates([None, "2024-01-01"], date=True) == "../2024-01-01"