In English, when telling time, a verb is used: "Now it is 9 o'clock." Additionally, when telling at what time, a preposition is used: "I will meet with the principal at 4 o'clock." In Chinese is a verb used when telling time (i.e.: 现在是九点 vs 现在几点), and is a preposition used when telling at what time in a sentence?
Use the verb "是" to tell 'what time it is'
e.g. 现在是九点 (it is nine O'clock)
Use the preposition "在" to tell 'at what time'
e.g. 在九点鐘 (at nine O'clock)
'at' in "I will meet with the principal at 4 o'clock." is "在" in Chinese
我會(I will) 在(at) 四點鐘(four o'clock) 會見 (meet) 校長 (the principal)”
preposition [at] + [time] = [在] + [時間]
At four o'clock = 在四點鐘
On Sunday = 在星期天
At the first of July = 在七月一號
In 2020 = 在二零二零年
You can use 於 (more literary) instead of 在 (more colloquial)