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Tips

Each guide is a handful of full sentences about one job. Read it, then open the matching calculator.

Reading a time zone result when clocks change
The hour gap can stay put while UTC offsets move. Use the date you mean.
Counting days between two dates
End minus start. Midnight when times are blank. January 1 to February 1 is 31 days.
US customary versus metric on this site
Inch, pound, Fahrenheit, and the US gallon first. Metric is a labeled switch.
Making a fair random list
One idea per line. Blank lines are dropped. Save the list before you click.
Choosing a password length
Entropy is length × log2(alphabet), not a 0–100 score. The string stays local.
Keeping aspect ratio when you resize
New height equals new width times height divided by width. 1920 × 1080 at 1280 is 720.
Hex versus RGB
#ff0000 is rgb(255, 0, 0). Six digits, or three integers from 0 through 255.
Keeping a short packing or decision list
Write the list in a note, then let the picker choose the next line.
Planning a call across three time zones
One instant, three clocks. Read the axis as a single mark.
Reading a countdown
Target minus this device’s clock. Convert a foreign clock before you type it.