When and where did "Five in the afternoon" and "Five o'clock" become "5 p.m."?

Consider this stanza from Byron's Don Juan:

What then? I do not know, no more do you.
And so good night. Return we to our story:
'T was in November, when fine days are few,
And the far mountains wax a little hoary,
And clap a white cape on their mantles blue;
And the sea dashes round the promontory,
And the loud breaker boils against the rock,
And sober suns must set at five o'clock.

The setting suns would suggest the author meant 5 p.m., and not 5 a.m.

To the best of my knowledge, with a few notable exceptions, abbreviations in general were not part of everyday speech before the Twentieth Century. (What forced them into everyday speech is a whole different story, I suppose).

I've done some research but, to my surprise, I couldn't find anything that would even remotely suggest when, where, and why the abbreviations for "post meridian" and "ante meridian" became standard everyday terms, replacing all those clunky yet, in my humble view, somewhat romantic, if not downright poetic, "half past eight in the evening," "two o'clock in the afternoon," etc, etc.

When?

Where?

Why?

Solution 1:

The classical Latin terms for the time before midday were ante meridiem (whence a.m.) and antemeridianus (whence antemeridian). After midday was post meridiem (whence p.m.), and pomeridianus or postmeridianus (whence pomeridian and postmeridian).

The earliest attestations in the OED are mostly from late 16th-century astrological works, which seems plausible: if anyone would adopt a classical term to describe the halves of the day, it would be those studying the sky. The earliest OED attestation to use of any of these terms with a time of day is from John Rolland's Ane treatise callit the court of Venus (1575):

About the thrid hour Pomeridiane.

Still, the contracted forms appear sooner than you might think, and not just for astrological use. The oldest use of p.m. attested in the OED is from 1642, in the Order of Assistance to Comm.:

Die Jovis 28. April. 1642. p. m. Ordered by the Lords and Commons in Parliament, That the abovesaid Order shall be forthwith printed and published.

A few years later, William Lilly uses ante meridiem and post meridiem in Christian astrology modestly treated of (1647)

If your hour of the day be in the morning, or as we say Ante Meridiem, or before noon.

I would erect a Figure of Heaven the sixt of January 1646, one hour thirty minutes afternoon, or P.M, that is Post Meridiem.

It is not possible to pinpoint an exact time when they became standard, but there are a number of OED examples of the abbreviated forms from the mid-18th century onwards.

Pocket watches and the pendulum clock were both popularized in the late 17th century. So, while this is purely speculative on my part, perhaps the arrival of cheaper, more accurate, and more accessible timekeeping increased the reporting of hour as opposed to vaguer notions of the day (e.g. a quarter past five rather than merely early in the morning). This would have revealed a need to distinguish between morning and evening more readily and succinctly.