fix freq='D' behaviour of standardized indices #5761

Job	Run time
Black (Python3.8)	58s
test-conda-Python3.9	6m 20s
test-py38-coverage-eofs (Python3.8)	4m 55s
test-${{ matrix.tox-env }} (Python${{ matrix.python-version }})	0s
test-conda-Python3.12	7m 9s
test-py39-coverage-sbck-eofs (Python3.9)	5m 54s
test-py310-coverage (Python3.10)	8m 19s
test-py311-coverage-sbck (Python3.11)	5m 3s
test-py312-coverage-numba (Python3.12)	5m 52s
test-notebooks_doctests (Python3.10)	6m 13s
test-offline-prefetch (Python3.11)	4m 31s
finish	15s
	55m 29s

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