Colloquially, if we assume that you have already know whether it's in the morning or in the afternoon, we normally say '现在(是)8点' for both situations. When you are talking with other people who is/are from different time zone, I would use '8 o'clock'/8点 and 20 o'clock/20点 rather than '8 AM' and '8 PM', because it's much easier to calculate time difference. In formal speeches, notices, newspapers, you'd better add '*上午*/*早*'(A.M.) or '*下午*/*晚*'(P.M.). It's 8 AM. 现在是上午8点。or 现在是早(上)8点。 It's 8 PM. 现在是下午8点。or 现在是晚(上)8点。

AM:上午 PM:下午 It is 8am :现在是上午八点

上午/早上 = am /eg.现在是上午（or 早上）8点//// 下午（about 1o'clock to when the sun goes down）and 晚上（when it's dark outside）=pm eg. 现在是下午3点（3 pm） 现在是晚上8点（8 pm）

AM = 上午= shàng-wǔ PM = 下午 = xià-wǔ