Add day_obs_offset class property

Default to 0 seconds offset.

python/lsst/pipe/base/_instrument.py

@@ -81,6 +81,10 @@ class Instrument(metaclass=ABCMeta):
     of the dataset type name, a tuple of dimension names, and the storage class
     name. If `None` the ingest system will use its default definition."""
 
+    day_obs_offset: 0
+    """Offset (in seconds) to apply when converting a ``day_obs`` value
+    to a timespan (`int`)."""
+
     def __init__(self, collection_prefix: str | None = None):
         if collection_prefix is None:
             collection_prefix = self.getName()