Could someone please let me know whether there is a rule concerning the use of periods with am and pm, and whether such a rule differentiates between quotation of the time in letters or numericals. It seems to be quite common to omit the period with numericals, as for example in 7 am. But is it also correct to write seven am ?
Thanks in advance for your help!
There is a lot of flexibility in how you write time.
Just be consistent.
We very rarely write the time in words. An exception are very formal wedding invitations, which would say "seven o'clock in the evening" or "nine o'clock in the morning."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.