FormatTime()/ParseTime() format specifier for RFC3339 date/time strings, with trailing zeros trimmed.
Declared in <absl/time/time.h>
extern
char const RFC3339_full[];
Note that RFC3339_sec[]matches an ISO 8601 extended format for date and time with UTC offset. Also note the use of "%Y": RFC3339 mandates that years have exactly four digits, but we allow them to take their natural width.