Why does 0.5 hours equal 30 minutes, not 50?
Half of anything is 50 percent. Half a dollar is 50 cents. So half an hour must be 50 minutes, right?
Wrong.
An hour is built from 60 minutes. Half of 60 is 30. Always.
The mistake starts the moment someone enters 0.5 on a timesheet and pictures a clock showing :50. That single slip overcounts by 20 minutes. Repeat it across a week of half-hour entries and the totals drift by hours.
Payroll systems use decimal hours precisely to avoid this. 30 minutes converts to 0.50. 15 minutes converts to 0.25. 45 minutes converts to 0.75. The decimal point describes a fraction of 60, never a minute count. Multiply the decimal by 60 to get the minutes. 0.5 × 60 = 30. No exceptions.
Does 1.5 hours mean 1 hour 50 minutes?
No. This error chains directly from the first. Someone sees 1.5 hours and writes 1:50. That is wrong by 20 minutes.
The whole number is the hours. The decimal is a fraction of the next hour. 1.5 means one full hour plus half of another. Half of 60 minutes is 30 minutes. So 1.5 hours equals 1 hour 30 minutes.
The same rule works for any decimal. 0.25 × 60 = 15 minutes. 0.75 × 60 = 45 minutes. 0.10 × 60 = 6 minutes. Never read the digits after the decimal point as minutes. They are not.
Over a full work week, five entries of 1.5 hours misread as 1:50 each add up to an extra 100 minutes that were never worked. That is enough to trigger a payroll audit or a client dispute.
Midnight: start of the day or end of it?
"Midnight Friday" is the most dangerous phrase in scheduling. It can mean the very start of Friday at 00:00. It can also mean the last moment of Friday at 23:59. People miss flights because of this. Deadlines get blown.
Always write 00:00 or 23:59. Never write "midnight."
ISO 8601, the international date and time standard, uses 00:00 for the start of a day and 24:00 for the end of the preceding day. In practice, 23:59 is clearer for deadlines. If you receive an invitation that says "midnight Friday," ask which side of the day they mean before you accept.
How many days are in a month?
January has 31 days. February has 28, or 29 in a leap year. March has 31. April has 30. A three-month span can be 89, 90, 91, or 92 days depending on which months and whether February gets an extra day.
Some payroll and billing systems use a 30-day month as an approximation. That works only if both parties agree to it in writing. For real-world planning, count actual calendar days.
The same caution applies to years. A year is 365 days, except when it is 366. Leap years fall in years divisible by 4, but century years must be divisible by 400. The year 2000 was a leap year. 2100 will not be.
Not every day has 24 hours
Most days run 24 hours. The exceptions hit twice a year.
On the day clocks spring forward, the day has 23 hours. On the day clocks fall back, the day has 25 hours.
In the United States and Canada, clocks spring forward on the second Sunday of March at 02:00 local time. They jump straight to 03:00. That day is 23 hours long. Clocks fall back on the first Sunday of November at 02:00, repeating the hour from 01:00. That day stretches to 25 hours.
In the United Kingdom, clocks spring forward on the last Sunday of March and fall back on the last Sunday of October.
A recurring event set for "every 24 hours" will land an hour early on spring-forward day and an hour late on fall-back day. If the event crosses a DST boundary, check the actual elapsed time.
When is London not 5 hours ahead of New York?
For most of the year, London runs on GMT (UTC+0) or BST (UTC+1), and New York runs on EST (UTC-5) or EDT (UTC-4). The standard gap is 5 hours.
But the gap shrinks and stretches during the DST transition weeks.
In March, the United States springs forward on the second Sunday. The United Kingdom waits until the last Sunday. For about two to three weeks, New York is already on EDT (UTC-4) while London is still on GMT (UTC+0). The difference drops to 4 hours.
In October, the United Kingdom falls back on the last Sunday. The United States does not fall back until the first Sunday of November. For about one week, London is on BST (UTC+1) and New York is still on EDT (UTC-4). The difference returns to 5 hours, then settles there after the US change.
During those windows, never assume the offset. Check both cities' current DST status.
Why does China use only one time zone?
China spans roughly five geographical time zones. From the western border to the eastern coast, solar noon varies by hours. Yet the entire country runs on a single clock: UTC+8, called Beijing Time.
There is no daylight saving time. China abolished DST in 1991. The clock never changes.
If you schedule a call with someone in Shanghai and someone in Kashgar, the time on the wall is identical in both cities. Local work and school schedules shift to match daylight, but the official hour does not.
Are UTC and GMT the same thing?
For everyday purposes, yes. The clock reads the same. The definitions are different.
GMT, Greenwich Mean Time, is a time zone. It is based on astronomical observation of the sun crossing the prime meridian. Earth's rotation is not perfectly constant, so GMT drifts.
UTC, Coordinated Universal Time, is an atomic time standard. It is kept by cesium atomic clocks and is extremely stable. Leap seconds are added to keep UTC within 0.9 seconds of astronomical time. The last leap second was added on 31 December 2016.
The gap is less than a second and irrelevant for daily planning. In computing, aviation and scientific work, UTC is the standard. GMT is a time zone that observes DST, becoming BST in summer. UTC never changes.