• fix parsing negative year and Unix time (%Y, %G, %s)
• fix formatting negative year, century, Unix time (%Y, %G, %C, %y, %g, %s)
• fix %g parsing to use the same two-digit year threshold 69 as %y
• fix %s formatting and parsing on 32-bit platforms
• support parsing time zone offset with %:::z
• improve performance of parsing/formatting compound directives