Writing & StyleStyle guide
AM vs. PM Formatting
AM and PM distinguish the two halves of a 12-hour day, but punctuation and capitalization are style choices. Microsoft house style uses uppercase “AM” and “PM,” preceded by a space; other style guides may use forms such as “a.m.” and “p.m.”
Follow your house style. Microsoft uses uppercase “AM” and “PM” with a preceding space, as in “10:45 AM”; other styles may use “a.m.” and “p.m.”
Key details
Core Issuecapitalization and punctuation of ante meridiem / post meridiem abbreviations
Microsoft Styleuppercase AM and PM, preceded by a space
Scopehouse-style dependent
Important caveats
Style Boundary
Do not present one publisher’s punctuation/capitalization as universal; follow the style guide governing the document.
Further guidance
Microsoft Style
Microsoft writes “AM” and “PM” in capital letters and places a space before them.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Date and time terms — Microsoft Style Guide (opens in a new tab)Microsoft · Microsoft house style for AM/PM and human-facing time notation