From 911bc4d87f27472d272bc62011e83986f2576ffe Mon Sep 17 00:00:00 2001
From: Stephan Finkensieper <stephan.finkensieper@dwd.de>
Date: Tue, 13 Feb 2024 13:26:52 +0000
Subject: [PATCH] Handle scans starting earlier than planned

---
 satpy/readers/ahi_hsd.py                 | 31 +++++++++++++++++++++---
 satpy/tests/reader_tests/test_ahi_hsd.py | 16 ++++++------
 2 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/satpy/readers/ahi_hsd.py b/satpy/readers/ahi_hsd.py
index 30e0f5e868..6b503148b2 100644
--- a/satpy/readers/ahi_hsd.py
+++ b/satpy/readers/ahi_hsd.py
@@ -792,10 +792,35 @@ def _modify_observation_time_for_nominal(self, observation_time):
                 stacklevel=3
             )
             return observation_time
+        timeline = self._get_closest_timeline(observation_time)
         dt = self._get_offset_relative_to_timeline()
-        return observation_time.replace(
-            hour=self.timeline.hour, minute=self.timeline.minute + dt//60,
-            second=dt % 60, microsecond=0)
+        return timeline + timedelta(minutes=dt//60, seconds=dt % 60)
+
+    def _get_closest_timeline(self, observation_time):
+        """Find the closest timeline for the given observation time.
+
+        Needs to check surrounding days because the observation might start
+        a little bit before the planned time.
+
+        Observation start time: 2022-12-31 23:59
+        Timeline: 0000
+        => Nominal start time: 2023-01-01 00:00
+        """
+        delta_days = [-1, 0, 1]
+        surrounding_dates = [
+            (observation_time + timedelta(days=delta)).date()
+            for delta in delta_days
+        ]
+        timelines = [
+            datetime.combine(date, self.timeline)
+            for date in surrounding_dates
+        ]
+        diffs = [
+            abs((timeline - observation_time))
+            for timeline in timelines
+        ]
+        argmin = np.argmin(diffs)
+        return timelines[argmin]
 
     def _get_offset_relative_to_timeline(self):
         if self.area == "FLDK":
diff --git a/satpy/tests/reader_tests/test_ahi_hsd.py b/satpy/tests/reader_tests/test_ahi_hsd.py
index 93089084c6..3b73521ee8 100644
--- a/satpy/tests/reader_tests/test_ahi_hsd.py
+++ b/satpy/tests/reader_tests/test_ahi_hsd.py
@@ -707,16 +707,16 @@ def test_areas(self, area, expected):
         ("timeline", "obs_start_time", "expected"),
         [
             (
-                "1200",
-                datetime(2023, 1, 1, 12, 0, 1),
-                {"tstart": datetime(2023, 1, 1, 12, 0, 0),
-                 "tend": datetime(2023, 1, 1, 12, 10, 0)}
+                "2350",
+                datetime(2022, 12, 31, 23, 50, 1),
+                {"tstart": datetime(2022, 12, 31, 23, 50, 0),
+                 "tend": datetime(2023, 1, 1, 0, 0, 0)}
             ),
             (
-                "1200",
-                datetime(2023, 1, 1, 11, 59, 59),
-                {"tstart": datetime(2023, 1, 1, 12, 0, 0),
-                 "tend": datetime(2023, 1, 1, 12, 10, 0)}
+                "2350",
+                datetime(2022, 12, 31, 23, 49, 59),
+                {"tstart": datetime(2022, 12, 31, 23, 50, 0),
+                 "tend": datetime(2023, 1, 1, 0, 0, 0)}
             ),
             (
                 "0000",