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Time and Calendar - Use of AM and PM

Grade 3ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A full day consists of 2424 hours. However, a standard circular clock face only shows numbers from 11 to 1212. Because of this, the hour hand must complete two full rotations around the clock every day to cover all 2424 hours.

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AM stands for \text{Ante Meridiem}, which means 'before midday'. This period covers the first 1212 hours of the day, starting at 12:0012:00 midnight and lasting until just before 12:0012:00 noon. Imagine this as the time from when you are deep in sleep until you are having your lunch at school.

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PM stands for \text{Post Meridiem}, which means 'after midday'. This period covers the second 1212 hours of the day, starting at 12:0012:00 noon and lasting until just before 12:0012:00 midnight. Picture the sun moving across the sky in the afternoon and the moon rising in the evening.

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The time 12:0012:00 noon is the exact middle of the day. On a timeline, this is the point where the AM period ends and the PM period begins. We usually call it '12 noon' or simply 'midday'.

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The time 12:0012:00 midnight is the beginning of a brand-new day. In a visual cycle, this is where the previous day's PM ends and the new day's AM starts. It is the boundary between two dates.

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To decide whether a time is AM or PM, look for daily activities. Events happening in the 'morning', during 'sunrise', or at 'breakfast' are AM. Events happening in the 'afternoon', 'evening', 'night', or during 'dinner' are PM.

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When a duration of time crosses 12:0012:00 noon, the label changes from AM to PM. For example, if you start an activity at 11:3011:30 AM and it lasts for one hour, it will end at 12:3012:30 PM.

📐Formulae

1textday=24texthours1 \\text{ day} = 24 \\text{ hours}

text1st12hours=12:00textmidnightto11:59textAM\\text{1st 12 hours} = 12:00 \\text{ midnight to } 11:59 \\text{ AM}

text2nd12hours=12:00textnoonto11:59textPM\\text{2nd 12 hours} = 12:00 \\text{ noon to } 11:59 \\text{ PM}

12texthours(AM)+12texthours(PM)=24texthours12 \\text{ hours (AM)} + 12 \\text{ hours (PM)} = 24 \\text{ hours}

💡Examples

Problem 1:

Rahul goes for his swimming class at 4:154:15 in the afternoon. Express this time using AM or PM.

Solution:

Step 1: Identify the period of the day mentioned. The problem says 'afternoon'.\nStep 2: Recall that any time after 12:0012:00 noon and before 12:0012:00 midnight uses the label PM.\nStep 3: Write the time with the correct label: 4:154:15 PM.

Explanation:

Because the activity takes place in the afternoon (post-midday), we use the suffix PM.

Problem 2:

A school trip starts at 10:3010:30 AM and lasts for 33 hours. What time does the trip end?

Solution:

Step 1: Start at the given time: 10:3010:30 AM.\nStep 2: Add 11 hour to get to 11:3011:30 AM.\nStep 3: Add another 11 hour to reach 12:3012:30. Since we have passed noon, the label changes from AM to PM. Time is now 12:3012:30 PM.\nStep 4: Add the final 11 hour (total 33 hours): 12:3012:30 PM +1texthour=1:30+ 1 \\text{ hour} = 1:30 PM.

Explanation:

To solve this, we add the hours step-by-step. Whenever we pass the 12:0012:00 noon mark, we must remember to switch the label from AM to PM.