docs: add comment explaining why astimezone(utc) is needed before stripping tzinfo

Co-Authored-By: Daryna Ishchenko <darina.ishchenko17@gmail.com>

Author: devin-ai-integration[bot]

airbyte_cdk/sources/file_based/file_based_stream_reader.py

@@ -118,6 +118,10 @@ def _parse_start_date(self, start_date_str: str) -> datetime:
             try:
                 return datetime.strptime(start_date_str, "%Y-%m-%dT%H:%M:%SZ")
             except ValueError:
+                # ab_datetime_parse may return a timezone-aware datetime (e.g. for inputs
+                # like "2025-01-01T00:00:00+05:30"). We convert to UTC first so the offset
+                # is applied correctly, then strip tzinfo to produce a naive UTC datetime
+                # compatible with RemoteFile.last_modified comparisons.
                 return (
                     ab_datetime_parse(start_date_str).astimezone(timezone.utc).replace(tzinfo=None)
                 )