fixed timedelta issue in longest_contiguous_slice (#725)

* fixed timedelta issue in longest_contiguous_slice

* made slice size calculation more comprehensible

Author: dennisbader

.pre-commit-config.yaml

@@ -3,4 +3,4 @@ repos:
     rev: 21.12b0
     hooks:
       - id: black-jupyter
-        language_version: python3.9
+        language_version: python3

darts/timeseries.py

@@ -1653,7 +1653,9 @@ def longest_contiguous_slice(self, max_gap_size: int = 0) -> "TimeSeries":
         max_slice_start = None
         max_slice_end = None
         for index, row in relevant_gaps.iterrows():
-            size = row["gap_start"] - curr_slice_start - self._freq
+            # evaluate size of the current slice. the slice ends one time step before row['gap_start']
+            curr_slice_end = row["gap_start"] - self.freq
+            size = curr_slice_end - curr_slice_start
             if size > max_size:
                 max_size = size
                 max_slice_start = curr_slice_start