When reading or writing times, two small marks often appear around numbers: the colon (:) and the period or full stop (.). In time notation, the colon separates hours from minutes and seconds, while the period mainly marks abbreviations such as a.m. and p.m. or decimal hours in technical contexts. These two punctuation marks work together to keep times precise and readable. This guide explains exactly what each mark is called, how to use them correctly, and how to avoid common mistakes.
What the Two Marks Are Called
The two recurring marks in time notation are the colon and the period. In digital and analog clocks, the colon separates hours from minutes 14:30, and it can also separate minutes from seconds 14:30:45. The period appears in lowercase form as a.m. and uppercase as p.m., indicating ante meridiem and post meridiem. Outside standard time telling, a decimal point may be used in technical or scientific reporting to express fractional hours. It is important to distinguish these from other similar marks, such as the comma, semicolon, and hyphen, which serve different roles.
How the Colon Structures Time
The colon is the standard separator used in nearly all digital and written time formats. It clearly divides the hour from the minute and, when necessary, the second. Because the colon is reserved primarily for time and ratios, it rarely causes ambiguity in everyday use. Style guides typically recommend using the numeric hour followed by a colon and two-digit minutes, with an optional second pair for precision.
Colon in 12-Hour and 24-Hour Time
In 12-hour clocks, the colon separates the hour from minutes before indicating a.m. or p.m., as in 9:15 a.m. or 9:15 a.m.:30 for precision. In 24-hour notation, the colon is used without a.m. or p.m., for example 09:15 for morning and 14:30 for early afternoon. Both systems rely on the same mark to organize time into clear, comparable units.
Seconds and Fine-Grained Time
When seconds are included, the colon separates minutes from seconds, producing a three-part structure HH:MM:SS. This format is common in timestamps, timers, and sports displays. The second colon does not indicate a new unit but simply extends the same hour:minute:second hierarchy. Using consistent spacing and colons helps avoid confusion, especially in logs and automated data.
How the Period Functions in Time Notation
The period has a different primary role: marking abbreviations and the end of sentences. In time notation, it appears in lowercase a.m. and uppercase p.m. to clarify whether the time is before or after midday. In scientific and technical writing, a decimal point can express fractions of an hour, such as 14.5 hours to indicate 14 hours and roughly 30 minutes. Unlike the colon, the period also ends declarative sentences.
a.m. and p.m. Explained
The strings a.m. and p.m. come from Latin, meaning ante meridiem (before midday) and post meridiem (after midday). The period in each abbreviation marks the end of the shortened form, distinguishing them from variables or simple letter pairs. Because these abbreviations are widely recognized, they remain the most common way to specify a 12-hour clock cycle without using 24-hour military time.
Decimal Hours and Technical Period Use
In fields such as accounting, payroll, and certain scientific calculations, time is sometimes written as a decimal number. Here the period acts as a decimal point, separating whole hours from fractional hours. For example, 2.75 hours corresponds to 2 hours and 45 minutes, since 0.75 of 60 minutes is 45. This notation keeps data compact for calculations but is less common in everyday time telling.
Style Rules and Common Pitfalls
Punctuation around times follows consistent conventions in most style guides. Use a colon between hours and minutes in digital notation, and a period after a.m. and p.m.. Avoid doubling colons or inserting extra spaces between the hour and minute marks. Be cautious with lowercase and uppercase forms, and always clarify whether a 12-hour or 24-hour system is intended. Consistent punctuation reduces ambiguity in schedules, logs, and timestamps.
Quick Reference: Correct and Incorrect Use
- Correct: 14:30, 9:05 a.m., 08:45 p.m.
- Correct (decimal): 14.5 hours to mean 14 hours and 30 minutes
- Incorrect: 14;30, 9:05a.m, 2:30 p.m : 00
- Avoid: ambiguous forms like 2:30 p or 14.30 when clarity is required
Practical Examples in Everyday Use
In everyday contexts, the colon dominates digital clocks, schedules, and timestamps, while the period anchors the a.m./p.m. abbreviations. A calendar entry may show 09:00–10:00 a.m., a timer might read 01:30:10, and a scientific dataset might list duration as 3.25 hours. Understanding how each mark functions helps you interpret and present time accurately across devices and documents.
Summary and Key Takeaways
The two marks used in time notation are the colon and the period. The colon structures hours, minutes, and seconds into clear intervals, while the period primarily appears in abbreviations and decimal representations. Proper use of these symbols reduces confusion and supports precise communication. Following standard style rules ensures that times remain legible and reliable, whether you are writing a schedule, a log, or a technical report.