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The 12-hour clock is a common timekeeping system used widely across the globe. It divides the 24 hours of a day into two cycles of 12 hours each.
The first cycle runs from midnight to midday (12 AM to 11:59 AM), known as "A.M.", which stands for "Ante Meridiem" or "before midday" in Latin. The second cycle runs from midday to midnight (12 PM to 11:59 PM), referred to as "P.M.", meaning "Post Meridiem" or "after midday".
This system helps people easily distinguish the time between morning and evening hours.

• AM: Midnight to 11:59 in the morning
• PM: Noon (midday) to 11:59 at night

This format is especially familiar in countries like the United States and Canada, among others. It's important to remember that unlike its 24-hour counterpart, which runs from 00:00 to 23:59, the 12-hour format cycles through two identical sets of numbers.

Greenwich Mean Time (GMT) is a historical time standard that was first established during the 19th century. It refers to the mean solar time at the Royal Observatory located in Greenwich, London.
GMT was widely adopted as the world's time standard and was crucial for navigation and timekeeping in the pre-digital era.
One key aspect of GMT is that it does not take into account daylight saving time changes, hence it remains consistent throughout the year.

• Historical Relevance: Used extensively in navigation.
• Location-Based: Based directly on solar time in Greenwich.
• No Daylight Saving Adjustment: Remains constant year-round.

Though now mostly replaced by Coordinated Universal Time (UTC), GMT still maintains relevance in certain contexts, such as in the aviation industry and the BBC World Service broadcasts.

Coordinated Universal Time (UTC) is the modern successor to GMT and serves as the primary time standard worldwide. It regulates international timekeeping and is a central component in the global synchronization of clocks.
Unlike GMT, UTC incorporates leap seconds to adjust for slight irregularities in Earth's rotation, ensuring precision.
Another distinction is that UTC remains unaffected by seasonal daylight saving time changes.

• Precision: Leap seconds ensure precise synchronization.
• International Standard: Used extensively across various time zones.
• Consistency: Remains unaffected by daylight saving changes.

UTC is critically important in industries requiring exact time coordination, such as telecommunications and satellite technology. It is also the backbone of time dissemination across global time zones.