Date Time Calculator

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Date Time Calculator
Calculate Duration Between Two DatesAdd or Subtract Time from a Date
Add (+)Subtract (-)
Result:
Enter dates to see calculation
function toggleInputs(){var type=document.getElementById('calc_type').value;if(type=='duration'){document.getElementById('duration_inputs').style.display='block';document.getElementById('addsub_inputs').style.display='none';}else{document.getElementById('duration_inputs').style.display='none';document.getElementById('addsub_inputs').style.display='block';}}function calculateResult(){var type=document.getElementById('calc_type').value;var showSteps=document.getElementById('steps').checked;var resultDiv=document.getElementById('answer');if(type=='duration'){var d1d=document.getElementById('d1_date').value;var d1t=document.getElementById('d1_time').value;var d2d=document.getElementById('d2_date').value;var d2t=document.getElementById('d2_time').value;if(!d1d||!d2d){alert('Please select both dates');return;}var start=new Date(d1d+'T'+d1t);var end=new Date(d2d+'T'+d2t);var diffMs=end-start;if(diffMs<0){var temp=start;start=end;end=temp;diffMs=Math.abs(diffMs);}var diffSecs=Math.floor(diffMs/1000);var days=Math.floor(diffSecs/86400);var hours=Math.floor((diffSecs%86400)/3600);var mins=Math.floor((diffSecs%3600)/60);var secs=diffSecs%60;var resStr=days+" days, "+hours+" hours, "+mins+" minutes, and "+secs+" seconds";if(showSteps){resStr="
Total Seconds: "+diffSecs.toLocaleString()+"
Breakdown:
– "+days+" full days
– "+hours+" hours
– "+mins+" minutes
– "+secs+" seconds

"+resStr;}resultDiv.innerHTML=resStr;}else{var bDate=document.getElementById('base_date').value;var bTime=document.getElementById('base_time').value;if(!bDate){alert('Please select a starting date');return;}var date=new Date(bDate+'T'+bTime);var op=document.getElementById('op_type').value=='add'?1:-1;var y=parseInt(document.getElementById('y_val').value)||0;var m=parseInt(document.getElementById('m_val').value)||0;var d=parseInt(document.getElementById('d_val').value)||0;var h=parseInt(document.getElementById('h_val').value)||0;date.setFullYear(date.getFullYear()+(y*op));date.setMonth(date.getMonth()+(m*op));date.setDate(date.getDate()+(d*op));date.setHours(date.getHours()+(h*op));var resStr=date.toDateString()+" "+date.toLocaleTimeString();if(showSteps){resStr="
Calculation: Start Date "+(op==1?"plus":"minus")+" "+y+"Y "+m+"M "+d+"D "+h+"H

"+resStr;}resultDiv.innerHTML=resStr;}}

Using the Date Time Calculator

Our date time calculator is a versatile tool designed for professionals and individuals who need precise temporal measurements. Whether you are calculating the exact duration of a project, determining your age down to the second, or projecting future deadlines, this tool provides instant and accurate results.

The calculator features two primary modes: Duration Calculation and Time Manipulation (Addition/Subtraction). This allows for a wide range of use cases from simple calendar math to complex scheduling adjustments.

Start Date/Time
The point of origin for your calculation. Defaults to midnight if no time is specified.
End Date/Time
The conclusion point. The tool calculates the inclusive or exclusive span between these two markers.
Operation (Add/Subtract)
Choose whether to move forward or backward in time from your base date.

How It Works

When you use the date time calculator, it processes dates using the standard JavaScript Date object, which follows the Gregorian calendar. The calculation involves converting dates into Unix timestamps (milliseconds since January 1, 1970) to perform math, then converting them back into human-readable formats.

Duration = (Date 2 – Date 1) / Time Constants

  • Days: 86,400,000 milliseconds
  • Hours: 3,600,000 milliseconds
  • Minutes: 60,000 milliseconds
  • Seconds: 1,000 milliseconds

Calculation Example

Scenario: You want to know exactly how long a project lasted that started on January 1, 2024, at 9:00 AM and ended on March 15, 2024, at 5:00 PM.

Step-by-step solution:

  1. Start Date: 2024-01-01 09:00:00
  2. End Date: 2024-03-15 17:00:00
  3. Calculate total milliseconds difference: 6,422,400,000 ms
  4. Convert to days: 74 days
  5. Remaining time: 8 hours
  6. Final Result: 74 days and 8 hours

Common Questions

Does this calculator account for leap years?

Yes, the date time calculator uses standard calendar logic that automatically accounts for February 29th during leap years. When you add a year to a date in a leap year, it correctly shifts based on the 366-day count.

What time zone does the calculator use?

The calculator operates using your local browser time zone settings. For the most accurate cross-time-zone results, it is recommended to convert your inputs to a single standard (like UTC) before calculating duration.

Can I calculate negative time?

In duration mode, the calculator will always return a positive absolute value between the two points. In add/subtract mode, you can move backward into past dates by selecting the 'Subtract' operation.

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