However, saying It's 2 by itself is uncommon: the o'clock is needed to specify that the current time is 2. Usage For business documents. Am/pm or 24-hour time are typically used. Telling time. For exact hours (1:00, 2:00, 3:00, ...) it is typical to say 1 o'clock when stating the time because as mentioned above, It's 1 by itself is ambiguous. More @Wikipedia
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