The 12-hour clock uses AM and PM to distinguish the two halves of a day. The 24-hour clock numbers the hours from 00 through 23, so the hour itself identifies which half of the day you mean. For example, 9:15 AM becomes 09:15, while 9:15 PM becomes 21:15. Both formats describe the same clock reading.
Neither format is a decimal-hours duration. The clock reading 17:30 means 5:30 PM; it does not mean someone worked 17.5 hours. To calculate worked time, you need a start, an end and, where relevant, dates and time zone information. Use the work hours calculator once those inputs are clear.
Convert AM times to the 24-hour clock
For AM times from 1:00 through 11:59, keep the hour and omit AM. Add a leading zero if you want a consistent two-digit display: 1:05 AM becomes 01:05, 8:40 AM becomes 08:40, and 11:59 AM becomes 11:59. The minutes do not change during the conversion.
The exception is the hour beginning at midnight. In standard 12-hour notation, 12:00 AM is midnight, which becomes 00:00. Likewise, 12:20 AM becomes 00:20 and 12:59 AM becomes 00:59. Keeping the 12 would move the reading into the noon hour and change its meaning by half a day.
When entering a schedule, consider writing “midnight” with the date as well as the numeric time. “Midnight Friday” can be interpreted as the start or end of Friday in casual language. A full timestamp such as “Friday, 00:00” makes the intended boundary clearer, provided everyone uses the same date convention.
Convert PM times to the 24-hour clock
For PM hours from 1 through 11, add 12 to the hour. Thus 1:00 PM becomes 13:00, 4:25 PM becomes 16:25, and 11:40 PM becomes 23:40. Leave the minutes unchanged. If seconds are present, leave those unchanged too: 4:25:30 PM becomes 16:25:30.
The noon hour is the exception. In standard notation, 12:00 PM is noon and remains 12:00. Similarly, 12:45 PM remains 12:45. Adding 12 to a noon-hour reading would incorrectly produce an hour beginning with 24, so apply the exception before using the general PM rule.
A useful check is that afternoon and evening times after the noon hour should begin with an hour from 13 through 23. Morning times should begin with 00 through 11. The hour 12 belongs to the noon hour, not to midnight. This check catches the most common mistakes without requiring a reference chart.
Convert back to AM and PM
For a 24-hour reading with hour 00, change the hour to 12 and use AM. Therefore 00:35 becomes 12:35 AM. For hours 01 through 11, keep the hour and add AM. For hour 12, keep 12 and add PM. For hours 13 through 23, subtract 12 and add PM.
Examples make the pattern easier to remember: 06:10 is 6:10 AM; 12:10 is 12:10 PM; 18:10 is 6:10 PM; and 23:10 is 11:10 PM. Notice that 06:10 and 18:10 share the same minutes but fall on opposite halves of the day.
For a mixed-format file, convert values into one consistent representation before sorting or calculating intervals. A text list containing “9:00 AM,” “13:00” and “2 PM” may not sort in chronological order. Standardizing the format also makes missing AM or PM markers easier to detect before anyone acts on the schedule.
Keep clock times separate from elapsed durations
Suppose a shift starts at 08:30 and ends at 17:15 on the same ordinary day. The elapsed span is eight hours and 45 minutes. Expressed as decimal hours, that duration is 8.75. The three values serve different purposes: 08:30 identifies the start, 17:15 identifies the end, and 8.75 measures the interval.
Subtracting the displayed numbers as if they were decimals gives the wrong method. For example, 17.15 − 8.30 = 8.85, but 8.85 decimal hours is eight hours and 51 minutes, not eight hours and 45 minutes. Clock minutes run from zero to 59, while decimal fractions divide an hour into hundredths and other base-ten parts.
To calculate manually, convert each clock reading to minutes after midnight: 17 × 60 + 15 = 1,035, and 8 × 60 + 30 = 510. The difference is 525 minutes. Dividing by 60 gives 8.75 hours. The decimal hours guide explains this second conversion in detail.
Record dates for overnight work
A schedule of 22:00 to 06:00 normally suggests an overnight interval, but the clock readings alone do not identify which dates are involved. With an end on the following day and no offset change, the span is eight hours. With a different end date, the span could be much longer.
Write both dates when a shift crosses midnight, spans several days or crosses a reporting boundary. Do not rely solely on the assumption that an end time earlier than the start must belong to tomorrow. That assumption can hide a reversed entry and cannot distinguish a genuine overnight shift from an accidental data entry error.
Local clock readings also need additional context around daylight saving changes. A clock can jump forward or repeat an hour. When exact elapsed time matters, preserve the applicable time zone offsets or another unambiguous timestamp representation. The overnight shift guide is a useful next step for organizing those inputs.
Choose a consistent schedule display
A team can use either format successfully if its conventions are clear. The 24-hour format removes the need for AM and PM markers, which can be useful in a compact roster. The 12-hour format may be more familiar to some readers. Choose based on the audience and keep the display consistent across calendars, messages and exports.
For a 12-hour schedule, include AM or PM on both endpoints where omission could cause confusion. “7–7” is ambiguous. “7:00 AM–7:00 PM” is explicit. For a 24-hour schedule, use consistent separators and leading zeros so a column is easy to scan: 07:00–19:00 is clearer than a mixture of 700, 7 and 19:00.
State the time zone when people in different locations share the schedule. A correct format conversion does not convert between time zones. Changing 3:00 PM to 15:00 preserves the local clock reading; it does not establish what time that event occurs for someone elsewhere. Keep that distinction visible in meeting and staffing records.
Check a schedule before sharing it
Review midnight and noon entries first because they are easy to reverse. Then inspect every overnight interval for an explicit date. Check that no 24-hour value contains a stray AM or PM marker and that every 12-hour value includes the intended marker. Confirm that minutes remain between 00 and 59.
Ask a second person to read a small sample as complete sentences: “The shift begins Tuesday at nine in the evening and ends Wednesday at five in the morning.” This exposes ambiguity that can be missed when both reviewers simply glance at the same row of numbers.
Finally, distinguish the display schedule from any calculation of breaks, payable time or staffing coverage. A clear start and end are necessary inputs, but they do not answer those separate questions. The Shift Schedules hub and Hours & Pay hub provide tools for the next stage of planning.
Frequently asked questions
Is 12 AM noon or midnight?
In standard notation, 12 AM is midnight and 12 PM is noon. Writing “noon” or a complete dated timestamp can reduce confusion when those boundary times appear in instructions or deadlines.
Is 24:00 the same as 00:00?
Some conventions use 24:00 to identify the end of a particular date, while 00:00 identifies the start of a date. Do not assume every system accepts 24:00. A clear alternative is 00:00 on the following date.
Does 14:30 mean 14.30 hours?
No. As a clock reading, 14:30 means 2:30 PM. As a duration explicitly labeled hours and minutes, it means 14 hours and 30 minutes, or 14.5 decimal hours. The field's meaning determines which interpretation applies.