Time Interval List Generator

9:00 AM to 5:00 PM every 30 minutes is 17 times

Eight hours split into 30-minute blocks gives 16 blocks. But a time sits at each edge of a block, so the list has one more entry than there are blocks: 17 times. 9:00 AM, 9:30 AM, 10:00 AM … 4:30 PM, 5:00 PM. It is the same reason a fence with 16 gaps needs 17 posts, and it is the most common slip when you build a schedule by hand.

The calculator starts at your first time, adds the interval again and again, and stops at the last time that does not pass your end time. The result is grouped by hour like a transit timetable, so a 9 AM row showing 00 and 30 means 9:00 AM and 9:30 AM. You can stop at an end time or after a set count, and you can leave the first time out of the list. Anything after midnight is tagged +1 day, +2 days and so on, counted from the day you started.

When you need a repeating time list

Plenty of tasks repeat at a fixed interval, but adding times by hand goes wrong near midnight or with an interval such as 1 h 30 min. These are cases you can load straight into the calculator.

• Appointment or interview slots: if slots start at 9:00 AM every 30 minutes and the office closes at 5:00 PM, the last slot starts at 4:30 PM. Set the end time to 4:30 PM and you get 16 slots. Try it
• Night patrol or inspection log: every 2 hours from 10:00 PM to 6:00 AM is 5 rounds: 10:00 PM, 12:00 AM, 2:00 AM, 4:00 AM and 6:00 AM, and the last four fall on the next day. Try it
• Shuttle or loop bus timetable: every 12 minutes from 7:00 AM to 9:00 AM is 11 departures, and the timetable view shows the minutes past each hour at a glance. Try it
• Measurement or observation times: eight readings every 15 minutes from 10:00 AM only needs the count mode, and the last time is 11:45 AM. Try it
• Instructions that say "every n hours": "every 8 hours" gives 3 times a day at the same times every day once you pick the first one. An interval that does not divide 24, such as every 7 hours, shifts the times from day to day. Try it

How to count the times: one more than the blocks

With both the first and the last time in the list, the count is the formula below. If the end time is not an exact multiple of the interval, the remainder of the division is dropped, and that remainder is the time left before the end time.

count = ⌊(end time − start time) ÷ interval⌋ + 1 · last time = start time + interval × (count − 1)

Leaving the first time out removes one entry. Which way to count depends on the job, but the iCalendar standard behind most calendar apps (RFC 5545) counts the start (DTSTART) as the first occurrence when it applies COUNT, and an UNTIL that lands exactly on a recurrence includes it. This calculator counts the same way by default, so an event built with that rule to repeat every 30 minutes, 17 times (FREQ=MINUTELY, INTERVAL=30, COUNT=17), produces the same 17 entries. Many calendar apps, though, do not offer minute-level repeats in their repeat settings; in that case, add the copied times one by one.

Adding the interval k times and crossing midnight

Adding the interval k times to the first time gives entry k + 1 of the list when the first time is included. To see whether it crosses midnight, turn the time into minutes since midnight, add, and divide by 1,440, the minutes in a day. The quotient is the number of days that passed and the remainder is the time of day.

time after k intervals (minutes) = start (minutes) + interval × k · day = quotient (÷ 1,440) · time of day = remainder

Take every 15 minutes from 11:40 PM. 11:40 PM is 23 × 60 + 40 = 1,420 minutes. Adding the interval twice gives 1,420 + 15 × 2 = 1,450 minutes. Dividing 1,450 by 1,440 gives quotient 1 and remainder 10, so it is 12:10 AM on the next day. The clock number got smaller, but the order did not flip: the date rolled over. The calculator tags such times +1 day and separates the days in the timetable. Try it

How many times a day, and when do the times repeat

A day has 1,440 minutes, so the number of times per day is 1,440 ÷ interval. If that is a whole number, the same times repeat every day. If not, the times drift, and the number of days before the original pattern returns is the interval divided by the greatest common divisor of the interval and 1,440.

times per day = 1,440 ÷ interval (min) · days until the same times = interval ÷ gcd(interval, 1,440)

Start every 7 hours at 8:00 AM and you get 8:00 AM, 3:00 PM and 10:00 PM, then 5:00 AM, 12:00 PM and 7:00 PM the next day, and 2:00 AM and 9:00 AM on the third. You need 7 × 24 = 168 hours before 8:00 AM comes around again, which is why the cycle is 7 days. Among whole-hour intervals, only 1, 2, 3, 4, 6, 8, 12 and 24 hours divide a day evenly.

The same list in Excel or Google Sheets

Excel and Google Sheets treat one day as 1. One hour is 1/24 and one minute is 1/1,440. The SEQUENCE function builds a column of numbers that grows by a fixed step from a start value, so putting your first time as the start and the interval as the step gives you a repeating time list.

ROUND snaps the minutes to a whole number so that a tiny floating-point error in a stored time cannot make INT drop one entry. A value past midnight is greater than 1, so an h:mm AM/PM format shows it as 12:10 AM again. To see which day it is, put =INT(A2) in the next column, or use the format [h]:mm to show 24:10. If the start and end are equal, MOD returns 0 and the count is 1, whereas this calculator reads equal times as "until the same time the next day".

Common mistakes

• Using the number of blocks as the count: 8 hours ÷ 30 minutes = 16 is the number of blocks. A schedule that shows both the first and the last time has 17 entries.
• Ending slots at closing time: for a 30-minute appointment to finish by 5:00 PM, the last start is 4:30 PM. Setting the end time to 5:00 PM adds a slot that starts when the office is closed.
• Reading 1.5 hours as 1 hour 50 minutes: 1.5 hours is 1 hour 30 minutes, which is 90 minutes. Enter 1 in the hours box and 30 in the minutes box, or 90 in the minutes box.
• Dropping the day after midnight: the 12:10 AM that follows 11:40 PM is on the next day. If you delete the +1 day tag when you paste the list into a calendar, the date moves back by one day.
• Treating "3 times a day" and "every 8 hours" as the same: every 8 hours runs through the night too, so 8:00 AM is followed by 4:00 PM and 12:00 AM. Three times between 8:00 AM and 10:00 PM means a 7-hour interval (8:00 AM, 3:00 PM, 10:00 PM).

Limits and notes

• Times are in whole minutes and the interval runs from 1 minute to 24 hours. One list holds up to 500 entries.
• No date is entered, so weekdays and holidays are not considered and every day counts as exactly 24 hours. Check separately for a daylight saving change or times in another time zone.
• The tool only shows a list and does not send reminders. If you need an alert at those times, paste the copied list into an alarm or calendar app.
• For a schedule where the interval changes or a break is inserted, build it in separate parts with equal intervals. A timetable with breaks between periods is better handled by the Bell Schedule Calculator.
• Everything is calculated inside this browser and nothing is sent to a server. The settings go into the address only when you press Copy link.

Sources

Sources checked on October 22, 2026. The spreadsheet formulas were calculated in LibreOffice 26.8 and returned the same times.

• 01RFC 5545 iCalendar, 3.3.10 Recurrence RuleCOUNT counts the start (DTSTART) as the first occurrence, UNTIL includes a matching recurrence, FREQ=MINUTELY with INTERVAL
• 02Microsoft Support: SEQUENCE function=SEQUENCE(rows, columns, start, step), rows and columns, start value and step
• 03Google Docs Editors Help: SEQUENCEThe same function in Google Sheets and its argument order
• 04NIST SP 811, Chapter 5: Units outside the SITable 6: 1 min = 60 s, 1 h = 60 min, 1 d = 24 h
• 05WHATWG HTML Standard: time input (type=time)The value is a 24-hour "hh:mm" string, while the displayed format may follow the device settings

Frequently asked questions

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