calendar: Use sensible error reporting in calendar_gregorian_to_offset().
[pspp] / src /
drwxr-xr-x   ..
-rw-r--r-- 21 .gitignore
-rw-r--r-- 486537 OChangeLog
-rw-r--r-- 947 automake.mk
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drwxr-xr-x - language
drwxr-xr-x - libpspp
drwxr-xr-x - math
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