Better late than never … no matter when on Wednesday the Tip of the Day is posted, it’s always on time! And speaking of time, here’s how to write it.
- Always use a.m. and p.m. For example: “The show started at 8 p.m.”
- Noon and midnight are preferred over 12 p.m. and 12 a.m.
- Don’t use a colon followed by double zeros. This is wrong: “The show started at 8:00 p.m.”
- For a time range, use a hyphen. Like this: “The performance is scheduled for 9:30-11 a.m.”
- As you may have noticed in the example above, you only have to reference a.m. or p.m. once. If the time range includes a.m. and p.m., use both. For example: “I was in the meeting from 10 a.m.-2 p.m.”
- It’s OK to use “o’clock” but it’s best used in prose, or long-form writing. For example: “It was dark and stormy, which was odd for 3 o’clock on a winter afternoon.”
ChanteSez.com 
Ohhh, I get to voice my pet peeve: timezone notation. Nobody every gets EST/EDT right, so DON’T BOTHER. Use ET, for Eastern Time — drop the middle letter. Daylight or Savings doesn’t matter, just say ET (or CT/MT/PT as appropriate). Between mid March and early November, if you specify the Eastern timezone as “EST” you are wroooooong, and this tip is for YOU.