Daylight saving time in the United States is the practice of setting the clock forward by one hour during the warmer part of the year, so that evenings have more daylight and mornings have less. Most areas of the United States currently observe daylight saving time (DST), the exceptions being Arizona (except for the Navajo Nation, which does observe daylight saving time),[1] Hawaii,[2] and the overseas territories of Puerto Rico, American Samoa, Guam, the Northern Mariana Islands, and the United States Virgin Islands.
Tết is celebrated on the same day as Chinese New Year, though exceptions arise due to the one-hour time difference between Hanoi and Beijing resulting in the alternate calculation of the new moon.
中国农历是一种以月亮和太阳运动为基础的历法,又称阴阳历。每个月由月亮的运动来确定,是为阴历,所以每月初一的公历日期不定。而节气则是由太阳的位置确定,系阳历,是故节气的公历日期基本固定。由于地球绕太阳公转的周期不可被月亮绕地球公转的周期整除,农历一段时间后就要加入闰月。虽说增加闰月的规则并不难描述,但若没有必须的天文知识则不易懂,且计算复杂不易应用。结果就是只有极少数人知道如何计算农历,印刷的日历成了大多数人知道如春节或中秋节等农历节日的公历日期的唯一来源。
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阴历的计算规则是在清朝顺治年间,即公元1645年,开始采用的。以上规则的一个重要特点就是理论上完全以天文观测为依据,其中没有任何数学关系。这和阳历,即格里历,完全是由数学关系确定的形成了鲜明的对比。当然,具体到未来阴历的计算,仍然需要月亮与地球运动的数学模型,而且精度高的这种数学模型相当复杂,在没有电脑的情况下,一般人根本不可能推算出精确的阴历来。而阳历的数学关系则非常简单,是不是闰年很容易计算。这又形成了另外一种鲜明的对比。
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