Time & Date Calculators

Why time and date arithmetic deserves its own calculators

Time is the one quantity everyone measures daily and almost nobody can do sums with. Dates live in a calendar where months hold 28, 29, 30 or 31 days and leap years arrive on a rule with two exceptions. Clock time runs in base 60, so 7:45 plus 0:30 is not 7:75. Time zones shift by whole hours, half hours and occasionally 45 minutes, and daylight saving quietly deletes or repeats an hour once a year. Every tool on this shelf exists because one of those quirks catches people out, and each one shows its working so you can see exactly how the answer was reached. All of them are free and run entirely in your browser.

The shelf splits naturally in two. Calendar tools answer questions about dates: how old, how many days, which weekday. Clock tools answer questions about durations: how long, how many hours, what does that add up to across a week. A few sit across both, and one deals with the strangest unit of all, the time zone.

Calendar questions: ages, gaps and deadlines

The gap between two dates

The Date Calculator is the general-purpose tool here. It does two jobs: it counts the days between any two dates, and it adds or subtracts days, weeks, months or years from a starting date, always telling you which weekday the result lands on. That second job is the one people get wrong by hand, because "add one month" is genuinely ambiguous when you start on the 31st; the calculator handles it calendar-correctly and shows how.

The Day Counter answers a more specific version of the same question. Instead of one raw total it splits the gap into weekdays, weekend days and a working-days figure, with a switch for whether the final day counts. That switch matters more than it looks: "10 days from Monday" means different things depending on whether Monday itself is day one, a classic fence-post problem that inflates or deflates deadlines by a day. If a contract, notice period or project plan talks about working days, this is the tool that matches its language.

Exact ages, done properly

The Age Calculator works out an age in years, months and days by walking the calendar forward from the date of birth, rather than dividing a day count by 365.25 and hoping. That distinction is why it can be exact: "38 years, 4 months and 12 days" is a statement about real calendar months, not an average. It also gives totals in days, weeks and hours, marks milestones, and counts down to the next birthday.

Which day of the week?

The Day of the Week Calculator names the weekday for any date from 1600 to 2400, and then does something unusual: it walks through the doomsday method step by step, the mental trick that lets you work out weekdays in your head. It finishes with a ten-year table showing which weekday a birthday or anniversary falls on, which is quietly useful for planning parties years ahead.

Clock questions: durations in base 60

The Time Calculator is the plain arithmetic tool for hours, minutes and seconds. You give it a list of durations to add or subtract and it shows the base-60 carrying explicitly, which is where hand calculations fall over; 90 minutes has to become 1 hour 30, not 0.90 of anything. A second tab multiplies or divides one duration by a plain number, so "six sessions of 1 hour 45" or "a third of 2 hours 20" stops being a puzzle.

The Time Duration Calculator answers "how long between these two moments?". It takes either two clock times or two full date-and-times and returns the gap in hours and minutes, decimal hours and total minutes. It handles two traps honestly: shifts that cross midnight, and the daylight saving changeover, where a wall-clock difference and an elapsed-time difference can disagree by an hour.

One idea underpins all of these: 7 hours 30 minutes is 7.5 decimal hours, not 7.3. Minutes are sixtieths, so you convert by dividing the minutes by 60. Both duration tools print the answer in both formats side by side precisely because payroll systems, invoices and spreadsheets tend to want decimal hours while people think in h:mm.

Timesheets, overtime and what the week pays

Two tools apply that clock arithmetic to work. The Hours Calculator takes a week of clock-in and clock-out times, subtracts breaks, copes with overnight shifts, and totals the week in both h:mm and decimal hours; add an hourly rate and it shows the gross pay too. It is the right tool when you simply need an accurate weekly total.

The Time Card Calculator goes a step further for anyone paid overtime. It splits the same punch data into regular and overtime hours at a threshold and multiplier you choose, prices each portion at your rate, and has a second-week toggle for fortnightly pay periods. If your question is not "how many hours?" but "what should this pay packet be?", start here, then carry the figure to the Take-Home Paycheck Calculator or the Salary Calculator to see what survives deductions.

Time zones and sleep: the clock meets the real world

The Time Zone Calculator converts a date and time between any two zones using your browser's own IANA time zone data, which means daylight saving rules are applied for the actual date you pick, not a generic offset. It shows each zone's current UTC offset, flags plainly when the conversion crosses into the day before or after, and includes a meeting planner strip that lines up working hours across as many as three zones so you can spot the overlap instead of guessing.

The Sleep Calculator applies duration arithmetic to bedtime. Sleep runs in cycles that average roughly 90 minutes, so it works backwards from your alarm (or forwards from lights-out) in whole cycles, adding a fall-asleep allowance you set, and presents a spread of sensible bedtimes rather than one false-precision answer. It is upfront that real cycles vary from about 80 to 120 minutes, so the times are starting points to test, not medical advice.

The mistakes these tools are built to catch

• Reading 7:30 as 7.3 hours. Half an hour is 0.5, and a timesheet built on this error drifts by minutes per shift and hours per month.
• Counting days inclusively when the situation is exclusive, or vice versa. The Day Counter's include-the-last-day switch exists for exactly this.
• Adding "a month" to the 31st and assuming the answer is obvious. It is not, and the Date Calculator shows the convention it applies.
• Forgetting daylight saving. An overnight duration across the spring changeover is an hour shorter on the clock than in elapsed time; the Time Duration and Time Zone calculators both make the distinction visible.
• Estimating age from "days divided by 365". Leap years make that increasingly wrong; walking the calendar, as the Age Calculator does, is the only exact method.

Frequently asked questions

What is the difference between the Date Calculator and the Day Counter?

The Date Calculator is the generalist: gaps between dates plus date arithmetic in days, weeks, months and years. The Day Counter specialises in one question, the day count itself, and breaks it into weekdays, weekends and working days with control over whether the end date counts.

Should I use the Hours Calculator or the Time Card Calculator?

Use the Hours Calculator when you need a clean weekly total from clock times and breaks. Use the Time Card Calculator when overtime is involved, because it splits hours at your threshold and applies the multiplier to the pay.

How do I convert minutes to decimal hours?

Divide the minutes by 60. So 45 minutes is 0.75 hours and 7 hours 20 minutes is 7.33 hours. Every duration tool on this page prints both formats so you never have to do the conversion by hand.

Are the time zone conversions accurate for daylight saving?

Yes, for the date you enter. The Time Zone Calculator uses the IANA zone database shipped with your browser, so a January conversion and a July conversion between the same two places can correctly differ by an hour.