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AM and PM

• AM stands for Ante Meridian.
• PM stands for Post Meridian.
• A day consists of 12 hours for AM, and 12 hours for PM.
• Some missed schedules (e.g. flights) due to the AM/PM confusion.
• Some medicine (e.g. pain relievers) to be taken in the evening and night has PM in the name.

Military

Military time uses the 24 hour notation.

Daylight Savings Time (DST)

• Daylight Savings Time (DST) is a controversial topic for a long time.
• In the US, some states do not implement DST.
• “Spring Forward. Fall Back” has some truth.
• Legislation changes have moved the start and end dates for DST.
• Some say that DST does not save energy, and the laws should be repealed.

Other Systems

• Some ancient Burmese Kingdoms employed the 60-hour day using the “water” clock.
  Day time can range from 24 to 36 hours. Ditto for Evening & Night time.
• Some early civilizations use the Sun Dial.
• The use of atomic clocks has to be supplemented by smart algorithms for idiosyncrasies introduced by DST, Leap second (to name a few).
• Depending on the culture, the Day of Week may start on a Sunday or a Monday.
• There are numerous software libraries dealing with Calendars, Dates, Times and Holidays.

U Khin Maung Zaw (EC76) wrote :

DST in US, especially at the Spring time when the clock moves an hour ahead (Spring Forward), always have some good stories. Some years back we used to have a large group of people playing soccer on Sunday early mornings, as is called pickup games. It never failed to see people showed up late, forgot to reset the clock, every year. Thank God! The change was on Sundays, or else there would be more interesting work-related stories.

Microsoft Windows have those dates usually baked into the system, as a result when the DST was extended 2008, they had to release a patch to fix the issue.