Use of Preposition or Verb with telling time
Use the verb &quot;是&quot; to tell 'what time it is'
e.g. 现在是九点 (it is nine O'clock)

~

Use the preposition &quot;在&quot; to tell 'at what time'
e.g. 在九点鐘 (at nine O'clock)

'at' in &quot;I will meet with the principal at 4 o'clock.&quot; is &quot;在&quot; 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)