Date and Time Calculator

The result is the start date and time plus or minus the duration

Forty-eight hours after Monday, June 15 at 2:00 PM is Wednesday, June 17 at 2:00 PM, the same clock time two days later. Add 1 day 2 hours 15 minutes to Thursday, May 28 at 12:30 PM and you land on Friday, May 29 at 2:45 PM. If a project is due Friday, June 19 at 6:00 PM and the job takes 9 hours 30 minutes, you have to start by 8:30 AM that morning.

The calculator above does this arithmetic for you. Its first screen shows the moment 48 hours from now, and turning on "Now" keeps the start glued to the current time, so "what time will it be in X hours" reads straight off the result card. The card shows the date, the weekday, the clock time and how many calendar days the result moved. The timeline underneath draws the duration across the days and midnights it crosses.

"Within 48 hours" and "within 2 days" do not end at the same moment

Return windows, appeal periods and filing deadlines are often written in hours or in days, and the two are counted differently. Some legal systems spell the difference out. One example is the rule federal courts in the United States use to count time (Federal Rules of Civil Procedure, Rule 6(a)). It is a court-procedure rule, not a general rule for contracts, store policies or state deadlines, but it shows the pattern clearly:

• Periods stated in hours: Rule 6(a)(2) says to "begin counting immediately on the occurrence of the event that triggers the period" to "count every hour, including hours during intermediate Saturdays, Sundays, and legal holidays", and "if the period would end on a Saturday, Sunday, or legal holiday, the period continues to run until the same time on the next day that is not a Saturday, Sunday, or legal holiday". A 48-hour period that starts at 2:00 PM is measured from that very moment.
• Periods stated in days: Rule 6(a)(1) says to "exclude the day of the event that triggers the period", "count every day, including intermediate Saturdays, Sundays, and legal holidays" and "include the last day of the period". A 2-day period that starts at 2:00 PM begins counting the next day and runs through the last day.
• When the last day is a weekend or holiday: under Rule 6(a)(1)(C) the period "continues to run until the end of the next day that is not a Saturday, Sunday, or legal holiday". Rule 6(a)(6) lists the legal holidays, and Juneteenth (June 19) is one of them.

These are the results for a period that starts Monday, June 15 at 2:00 PM. The day-based deadline runs about 10 hours longer. Under Rule 6(a)(4), unless a statute, local rule or court order sets a different time, the last day ends at midnight in the court's time zone for electronic filing, and when the clerk's office is scheduled to close for filing by other means. This calculator gives you the hour-based answer: the moment a number of hours after the start. For a day-based deadline, find the last day yourself and check what time it ends. Contracts, store return policies, state courts and agencies each set their own method, so read the document that governs your deadline first.

Counting by hand goes wrong in everyday situations too: when a night shift ends, when a rental car is due back, when an 8-hour reminder fires next, or when you work backward from a flight and cross midnight and a weekday at the same time.

How the calculation works

1. Weeks and days are added on the calendar: 1 week is 7 days, and N days later is the same clock time on the Nth day. The calendar handles month ends and leap years, so 1 day after October 31 is November 1 and 1 day after February 28, 2028 is February 29.
2. Hours and minutes are added as real elapsed time: 1 hour is 60 minutes and 1 day is 24 hours. Add 1 hour 20 minutes to 10:50 PM and the minutes reach 70, which carries an hour, and the clock passes midnight to 12:10 AM the next day.
3. Subtraction works in the same order, backward: 1 hour before 12:30 AM is 11:30 PM the day before. Don't type a negative number in the duration boxes; switch the direction to "− earlier" instead.
4. Check the weekday and the date change: "+2 days" on the result card means the date moved twice, and the rows below say how many midnights were crossed.

Formula result = start ± (weeks × 7 + days) days on the calendar, then ± (hours × 60 + minutes) minutes

Adding the larger unit, days, first and the hours and minutes afterward follows the iCalendar standard (RFC 5545). It separates days and weeks, whose length depends on where they fall on the calendar, from hours, minutes and seconds, which are exact. For exact durations it states: "When computing an exact duration, the greatest order time components MUST be added first, that is, the number of days MUST be added first, followed by the number of hours, number of minutes, and number of seconds." In a time zone and on dates with no clock change, a day is always 24 hours, so the order doesn't change the answer.

What time is it N hours from now?

To turn hours into days, divide by 24: the whole number is days and the remainder is hours. The right-hand column is the result for a start of Monday, June 15 at 9:00 AM.

168 hours is exactly one week, so you land on the same weekday at the same time. A duration that is not a multiple of 24 hours changes the clock time: 100 hours is 4 days plus 4 more hours, which puts you at 1:00 PM.

Worked examples

Every value below comes from the same calculator. Subtracting answers "when do I have to start?" and adding answers "when does it end?"

Many stores count a return window in days ("within 30 days of delivery") rather than hours, so check how the retailer's policy words it before you rely on a clock time. For a flight that crosses time zones, use a time zone calculator to get the local time at the other end instead of this tool.

When a day is not 24 hours: daylight saving time

In a time zone with no clock change, 1 day and 24 hours are always the same. In the United States the clocks change twice a year, so on those two dates a calendar day lasts 23 or 25 hours. The IANA time zone data puts the 2026 changes at March 8 and November 1. In the United Kingdom the clocks change on March 29 and October 25. Here is how New York time behaves:

On November 1, 2026 daylight saving time ends and the clocks fall back an hour; on March 8 it begins and they spring forward an hour. The calculator follows your device's time zone, so there "1 day" means the same clock time the next day, "24 hours" means 24 hours that really elapse, and a line under the result tells you the clock changed and how much real time passed.

Common mistakes

• Adding minutes like decimals: 1 hour 50 minutes plus 20 minutes is 2 hours 10 minutes, not 1 hour 70 minutes. Clock times work the same way: 9:50 AM + 20 minutes is 10:10 AM.
• Crossing midnight and keeping the date: 11:00 PM + 3 hours is 2:00 AM the next day. Going backward, 1 hour before 12:30 AM is the previous day.
• 12:00 AM and 12:00 PM: 12:00 AM is midnight, the start of the day, and 12:00 PM is noon. So 12:30 AM is 30 minutes after midnight. The result row shows the time in both 12-hour and 24-hour form.
• Treating "2 days" as 48 hours: as the table above shows, a day-based period that excludes the first day and runs through the last is longer than 48 hours. Reading "within 48 hours" as "by the day after tomorrow" can make you late.
• Ignoring the clock change: across the dates in the daylight saving table, "24 hours later" and "tomorrow at the same time" are different moments.

Limits and cautions

• Results use the time zone set on this device. To find local times in another city, use a time zone tool such as the World Meeting Time Calculator or the Airport Time Zone & Flight Time Calculator.
• The calculation is to the minute, without seconds, and the start date can be any date from year 1000 to 9999.
• It does not skip weekends or holidays to count business days, and it does not add months or years. For date-only work, use the Date Calculator & Date Difference tool.
• To find how much time passed between two moments, use the Time Duration Calculator.
• It does not decide whether a legal or contractual deadline is valid, or whether the last day is a legal holiday. For anything important, follow the date and time given by the court, agency or other party.

Sources

• 01Legal Information Institute (Cornell Law School): Federal Rules of Civil Procedure, Rule 6, Computing and Extending Time6(a)(1) days or longer units: exclude the triggering day, count every day, include the last day. 6(a)(2) hours: begin counting immediately, count every hour, and if the period would end on a Saturday, Sunday or legal holiday, continue to the same time on the next day that is not. 6(a)(4) when the last day ends. 6(a)(6) legal holidays. A rule for federal civil court deadlines, quoted as one example
• 02IETF RFC 5545: iCalendar, 3.3.6 Duration · 3.6.1 Event ComponentWeeks and days are nominal durations, hours, minutes and seconds are exact, and days are added first. An event with no DTEND ends at the same moment it starts
• 03BIPM: The International System of Units (SI Brochure, 9th edition)Table 8: 1 min = 60 s, 1 h = 60 min = 3600 s, 1 d = 24 h = 86 400 s
• 04IANA: Time Zone DatabaseAmerica/New_York changes on March 8 and November 1, 2026; Europe/London on March 29 and October 25, 2026 (the data browsers and Python zoneinfo use)

Frequently asked questions

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