Add method to translator for calculating observing day offset

This allows the base translator to calculate the observing day.
Need to change some of the dates in tests now that the offset
is being included properly.

python/lsst/obs/lsst/translators/lsst.py

@@ -204,6 +204,25 @@ def search_paths(self):
         """
         return [os.path.join(obs_lsst_packageDir, "corrections")]
 
+    @classmethod
+    def observing_date_to_offset(cls, observing_date: astropy.time.Time) -> astropy.time.TimeDelta | None:
+        """Return the offset to use when calculating the observing day.
+
+        Parameters
+        ----------
+        observing_date : `astropy.time.Time`
+            The date of the observation. Unused.
+
+        Returns
+        -------
+        offset : `astropy.time.TimeDelta`
+            The offset to apply. The default implementation returns a fixed
+            number but subclasses can return a different value depending
+            on whether the instrument is in the instrument lab or on the
+            mountain.
+        """
+        return cls._ROLLOVER_TIME
+
     @classmethod
     def compute_detector_exposure_id(cls, exposure_id, detector_num):
         """Compute the detector exposure ID from detector number and
@@ -807,10 +826,7 @@ def to_observing_day(self):
             self._used_these_cards("DAYOBS")
             return int(self._header["DAYOBS"])
 
-        # Calculate it ourselves correcting for the Rubin offset
-        date = self.to_datetime_begin().tai
-        date -= self._ROLLOVER_TIME
-        return int(date.strftime("%Y%m%d"))
+        return super().to_observing_day()
 
     @cache_translation
     def to_observation_counter(self):

python/lsst/obs/lsst/translators/lsstCam.py

@@ -13,6 +13,9 @@
 __all__ = ("LsstCamTranslator", )
 
 import logging
+
+import pytz
+import astropy.time
 import astropy.units as u
 
 from astro_metadata_translator import cache_translation
@@ -25,6 +28,8 @@
 # Normalized name of the LSST Camera
 LSST_CAM = "LSSTCam"
 
+_LSST_CAM_SHIP_DATE = 202406
+
 
 def is_non_science_or_lab(self):
     """Pseudo method to determine whether this is a lab or non-science
@@ -157,3 +162,32 @@ def to_physical_filter(self):
             joined = joined.removesuffix("~empty")
 
         return joined
+
+    @classmethod
+    def observing_date_to_offset(cls, observing_date: astropy.time.Time) -> astropy.time.TimeDelta | None:
+        """Return the offset to use when calculating the observing day.
+
+        Parameters
+        ----------
+        observing_date : `astropy.time.Time`
+            The date of the observation. Unused.
+
+        Returns
+        -------
+        offset : `astropy.time.TimeDelta`
+            The offset to apply. During lab testing the offset is Pacific
+            Time which can mean UTC-7 or UTC-8 depending on daylight savings.
+            In Chile the offset is always UTC-12.
+        """
+        # Timezone calculations are slow. Only do this if the instrument
+        # is in the lab.
+        if int(observing_date.strftime("%Y%m")) > _LSST_CAM_SHIP_DATE:
+            return cls._ROLLOVER_TIME  # 12 hours in base class
+
+        # Convert the date to a datetime UTC.
+        pacific_tz = pytz.timezone("US/Pacific")
+        pacific_time = observing_date.utc.to_datetime(timezone=pacific_tz)
+
+        # We need the offset to go the other way.
+        offset = pacific_time.utcoffset() * -1
+        return astropy.time.TimeDelta(offset)

tests/test_translator.py

@@ -414,7 +414,7 @@ def test_phosim_translator(self):
                            observation_id="204595",
                            observation_type="science",
                            observation_reason="phosim",
-                           observing_day=20221004,
+                           observing_day=20221005,
                            physical_filter="i",
                            pressure=520.0*cds.mmHg,
                            relative_humidity=40.0,
@@ -932,7 +932,7 @@ def test_imsim_translator(self):
                            observation_id="3010002",
                            observation_type="science",  # The header is wrong
                            observation_reason="imsim",
-                           observing_day=20211231,
+                           observing_day=20220101,
                            physical_filter="i",
                            pressure=None,
                            relative_humidity=40.0,
@@ -963,7 +963,7 @@ def test_imsim_translator(self):
                            observation_id="204595",
                            observation_type="science",  # The header is wrong
                            observation_reason="imsim",
-                           observing_day=20221004,
+                           observing_day=20221005,
                            physical_filter="i_sim_1.4",
                            pressure=None,
                            relative_humidity=40.0,
@@ -994,7 +994,7 @@ def test_imsim_translator(self):
                            observation_id="5000007",
                            observation_type="flat",
                            observation_reason="imsim",
-                           observing_day=20220805,
+                           observing_day=20220806,
                            physical_filter="i",
                            pressure=None,
                            relative_humidity=40.0,
@@ -1025,7 +1025,7 @@ def test_imsim_translator(self):
                            observation_id="4010003",
                            observation_type="science",  # The header is wrong
                            observation_reason="imsim",
-                           observing_day=20211231,
+                           observing_day=20220101,
                            physical_filter="i",
                            pressure=None,
                            relative_humidity=40.0,
@@ -1084,7 +1084,7 @@ def test_ts3_translator(self):
                            observation_id="ITL-3800C-098_lambda_flat_1000_067_20160722020740",
                            observation_type="flat",
                            observation_reason="lambda",
-                           observing_day=20160721,
+                           observing_day=20160722,
                            physical_filter="550CutOn",
                            science_program="2016-07-22",
                            visit_id=201607220607067)),