Words

a.m. / p.m.

a.m. - "ante meridiem" is Latin for "before noon"
p.m. - "post meridiem" is Latin for "after noon"

You can write "a.m." and "p.m." as lowercase letters with periods after them or as small capitals without periods. There should be a space between the time and the "a.m." or "p.m." that follows. Although small capitals used to be preferred, it's now more common to see lowercase letters followed by periods ("a.m." and "p.m.").

ref: https://www.quickanddirtytips.com/education/grammar/daylight-saving-time-0