Unix 时间戳
这将返回 Day.js 对象的 Unix 时间戳(自 Unix 纪元以来的秒数)。
¥This returns the Unix timestamp (the number of seconds since the Unix Epoch) of the Day.js object.
dayjs('2019-01-25').unix() // 1548381600
该值被取整到最接近的秒,并且不包括毫秒分量。
¥This value is floored to the nearest second, and does not include a milliseconds component.