Abbreviations for time read from a clock begin with small letters. Those for months and days of the week begin with capital letters. They all end with a period.
These abbreviations are used in charts, calendars, lists, informal writing, and the like. Spell the words out in standard formal writing.
May, June, and July are not normally abbreviated, though sometimes Jun. and Jul. are used when space is limited to three letters.
Clock time: sec. min. hr. (sometimes h.)
Days of the Week: Mon. Tue. (Tues.) Wed. Thu. (Thurs.) Fri. Sat. Sun.
Months: Jan. Feb. Mar. Apr. Aug. Sep. (Sept.) Oct. Nov. Dec.
A.M. and P.M. may either be written in all capital letters or all lower case, but choose one style and stick with it. B.C, B.C.E., and A.D. are always capitalized. Each letter in all five abbreviations is followed by a period.
These abbreviations may be used in all types of writing but only with numbers or a numerical reference.
If you wish to use B.C., B.C.E., or A.D. in a sentence referring to a century, the abbreviation follows the century.
B.C. and B.C.E. always follow the date; A.D. may either precede or follow a numerical date.
None of these abbreviations are separated by commas.
Incorrect: We will meet in the p.m.
(OK informally; not standard use, no number.)
Correct: We will meet at 1:15 p.m.
Correct: Alexander ruled in the fourth century B.C.
(Abbreviation always follows century reference.)
Correct: Charlemagne was crowned in A.D. 800.
Correct: Charlemagne was crowned in 800 A.D.
(Either way is OK for A.D. and a number.)
Since a.m. means "before noon" and p.m. means "after noon," use no other expression of time of day with them.
Incorrect: He arrived at 10 p.m. in the evening.
(In the evening is redundant)
Correct: He arrived at 10 p.m.
Correct: He arrived at 10 in the evening.