Noon and midnight in 12-hour time
12 AM and 12 PM label different boundaries; neither should be guessed from the number 12 alone.
Two one-hour examples
| 12-hour input | 24-hour equivalent | Duration |
|---|---|---|
| 12:00 AM–1:00 AM | 00:00–01:00 | 01:00 |
| 12:00 PM–1:00 PM | 12:00–13:00 | 01:00 |
Keep the date next to midnight
12:00 AM marks the start of the date you select. If a shift runs from 11:00 PM to 1:00 AM, its end date must be the following date to describe that two-hour overnight record. Changing only the AM/PM selector is not a substitute for confirming the date.
Equal times need an explicit span
08:00 to 08:00 on the same date is not automatically twenty-four hours. To record a full day, enter today’s start and tomorrow’s end. This avoids interpreting an unfinished or mistaken row as a completed 24-hour shift. A display preference between 12-hour and 24-hour time must not change the stored dates or minute values.
Use this example
Open related toolFrequently asked questions
Why does the loaded example total 02:00?
It contains two separate one-hour shifts: 12:00 AM–1:00 AM and 12:00 PM–1:00 PM on the same date. Each contributes 01:00.
Is 12:00 AM the end or the start of its selected date?
It is 00:00 at the start of that date. Use the following date explicitly when midnight belongs to the end of an overnight record.
Sources and method
- NIST SP 330, Section 4, Table 8Time-unit definitions only; not a payroll authority or product endorsement.
- Original worked calculationSynthetic example data, explicit input rules and direct integer-minute arithmetic.
Keep going
Time card calculator
Build a dated time card with multiple shifts, paid and unpaid breaks, local projects, CSV import and PDF or JSON exports.
Dates, clock times and durations
Read supported dates, 12- and 24-hour clock times, long durations, decimal hours and explicit fractional-minute handling.
Worked time-card examples
Browse twelve checked examples of split shifts, overnight breaks, date boundaries, decimal durations and file review.