Fix custom date format parse #1052

Job	Run time
check-vegafusion-python	10m 38s
build-vegafusion-wasm	8m 52s
build-vegafusion-python-linux-64	14m 24s
build-vegafusion-python-linux-arm64	15m 29s
build-vegafusion-python-osx-64	26m 7s
build-vegafusion-python-osx-arm64	18m 8s
build-vegafusion-python-win-64	33m 38s
build-vegafusion-server-linux-64	13m 41s
build-vegafusion-server-linux-arm64	14m 35s
check-rust	6m 7s
test-rust (ubuntu-20.04)	20m 25s
build-vegafusion-server-osx-64	31m 43s
test-rust (windows-2022)	36m 41s
build-vegafusion-server-osx-arm64	12m 31s
test-rust (macos-14)	13m 1s
build-vegafusion-server-win-64	32m 15s
test-vegafusion-python-linux-64	26m 20s
test-vegafusion-python-osx-arm64	49s
test-vegafusion-python-win-64	1m 56s
bundle-vegafusion-python-wheels	7s
bundle-vegafusion-server-builds	6s
	5h 37m 33s

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