Dates and time zones
The Date Difference Calculator gives both answers people actually want: an exact calendar difference in years, months and days, and a plain total in days, weeks and hours, plus weekday and weekend counts. It advances whole months with clamping rather than dividing by thirty, which is why 31 January to 1 March comes out as one month and one day instead of the negative figure a naive calculation produces. The Time Zone Converter compares cities side by side with daylight saving applied for the date you pick — which matters for the few weeks each year when one country has changed its clocks and another has not.
Money maths
The Compound Interest Calculator steps month by month rather than using the textbook formula, because that formula is only correct when contributions and compounding happen at the same frequency — and people usually save monthly into accounts that compound quarterly. A year-by-year table shows the crossover point where growth starts outpacing contributions. The Tip, Discount and Tax Calculator covers three everyday sums including working backwards from a tax-inclusive price, which is a genuinely different calculation from subtracting the percentage. The Invoice Generator produces a clean PDF with no watermark or monthly cap.
Focus, notes and timing
The Online Notepad autosaves as you type into your browser's own storage, with no account and nothing uploaded — instant, but device-local, so download anything worth keeping. The Pomodoro Timer runs against the wall clock rather than counting ticks, which matters because browsers throttle background-tab timers and a naive countdown genuinely loses time precisely when you are working. The Stopwatch and Timer covers plain timing with lap splits.
Picking fairly, and measuring things
The Spinner Wheel draws its winner with cryptographic randomness before the animation starts, then animates to land on it — the opposite of reading the result off wherever a decaying spin happens to stop, which is subtly biased. Dice and Coin Flip uses rejection sampling, because a plain modulo of a random number very slightly favours lower faces. Elsewhere, the Aspect Ratio Calculator scales dimensions without distortion in either direction, the Contrast Checker tests colours against WCAG and suggests the closest passing shade, and Roman Numerals and Number to Words handle the two notations people still write by hand.