Calculating clock time involves measuring the interval between two points in time or determining a future/past time based on a specific duration. This is essential for payroll, tracking project hours, or planning schedules.
The Duration Formula
To find the difference between two times manually, follow these steps:
Convert to 24-hour format: If it's PM, add 12 to the hour (except for 12 PM). If it's 12 AM, set the hour to 0.
Subtract the Start from the End: Subtract minutes from minutes and hours from hours.
Handle Borrowing: If the end minutes are less than start minutes, "borrow" 60 minutes from the hour column.
Overnight Handling: If the end time is on the next day, add 24 hours to the end time before subtracting.
Examples of Clock Time Calculation
Start Time
End Time
Logic
Total Duration
8:30 AM
5:15 PM
17:15 minus 08:30
8 hours, 45 minutes
10:00 PM
2:30 AM
26:30 minus 22:00
4 hours, 30 minutes
11:45 AM
12:15 PM
12:15 minus 11:45
0 hours, 30 minutes
Why Precision Matters
Whether you are calculating billable hours or kitchen prep times, precision is key. A simple mistake in "borrowing" minutes (forgetting that 1 hour = 60 minutes, not 100) can lead to significant errors in timekeeping. Our calculator automates this logic to ensure 100% accuracy for 12-hour and 24-hour cycles.
function calculateDuration() {
var startHr = parseInt(document.getElementById("startHour").value);
var startMn = parseInt(document.getElementById("startMin").value) || 0;
var startAp = document.getElementById("startAmPm").value;
var endHr = parseInt(document.getElementById("endHour").value);
var endMn = parseInt(document.getElementById("endMin").value) || 0;
var endAp = document.getElementById("endAmPm").value;
if (isNaN(startHr) || isNaN(endHr)) {
alert("Please enter valid hours.");
return;
}
// Convert Start to 24h minutes
if (startAp === "PM" && startHr !== 12) startHr += 12;
if (startAp === "AM" && startHr === 12) startHr = 0;
var startTotalMinutes = (startHr * 60) + startMn;
// Convert End to 24h minutes
if (endAp === "PM" && endHr !== 12) endHr += 12;
if (endAp === "AM" && endHr === 12) endHr = 0;
var endTotalMinutes = (endHr * 60) + endMn;
// If end time is earlier than start time, assume it's the next day
if (endTotalMinutes = 12) ? "PM" : "AM";
var finalHr12 = finalHr24 % 12;
if (finalHr12 === 0) finalHr12 = 12;
var displayMn = (finalMn < 10) ? "0" + finalMn : finalMn;
document.getElementById("addResult").innerHTML = finalHr12 + ":" + displayMn + " " + finalAp;
document.getElementById("addResultBox").style.display = "block";
}