时间格式化

• d3.time.format - create a new local time formatter for a given specifier.
• format - format a date into a string.
• format.parse - parse a string into a date.
• d3.time.format.multi - create a new local multi-resolution time formatter.
• d3.time.format.utc - create a new UTC time formatter for a given specifier.
• d3.time.format.iso - the ISO 8601 UTC time formatter.