在OP的编辑之后：
var now = new Date();var start = new Date(now.getFullYear(), 0, 0);var diff = now - start;var oneDay = 1000 * 60 * 60 * 24;var day = Math.floor(diff / oneDay);console.log('Day of year: ' + day);编辑：上面的代码会失败时，
now是3月26日和10月29日之间的日期
now的时间是凌晨1点之前（如零时59分59秒）。这是因为该代码未考虑夏令时。
var now = new Date();var start = new Date(now.getFullYear(), 0, 0);var diff = (now - start) + ((start.getTimezoneOffset() - now.getTimezoneOffset()) * 60 * 1000);var oneDay = 1000 * 60 * 60 * 24;var day = Math.floor(diff / oneDay);console.log('Day of year: ' + day);
