Fix time indexing regression in `convert_calendar` #13121

Job	Run time
detect ci trigger	3s
Event File	2s
ubuntu-latest py3.9	12s
ubuntu-latest py3.12	14s
macos-latest py3.9	14s
macos-latest py3.12	16s
windows-latest py3.9	11s
windows-latest py3.12	10s
ubuntu-latest py3.9 bare-minimum	14s
ubuntu-latest py3.9 min-all-deps	14s
ubuntu-latest py3.11 all-but-dask	13s
ubuntu-latest py3.12 flaky	14s
	2m 17s

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