fix GTFS reference date for time period begin #502

Job	Run time
formatting	1m 4s
msvc (Debug)	3m 18s
macos (macos-x86_64, On)	4m 50s
msvc (Release)	4m 46s
linux (linux-amd64-release, linux-amd64)	25s
macos (macos-arm64)	4m 56s
linux (linux-x86-release, linux-x86)	25s
linux (linux-arm-release, linux-arm, qemu-arm-static)	26s
linux (linux-arm64-release, linux-arm64, qemu-aarch64-static)	26s
linux (clang-tidy)	1m 37s
linux (linux-sanitizer)	1s
linux (linux-debug, valgrind --leak-check=full --error-exitcode=1)	19s
	22m 33s

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