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Part of the CalcProMaster guides library · Dates, bills & quick daily math
The calculations nobody teaches: exact age in years-months-days, days between dates, an electricity bill decoded line by line, fuel split across a road trip. Small math, daily use — these guides make each one take ten seconds.
Date math has exactly two traps. Age: you completed whole years on your last birthday — the formula subtracts month and day before borrowing from the year, which is why it never overcounts. Leaplings (29 February birthdays) age on 28 Feb or 1 Mar depending on convention; pick one and stay consistent.
Electricity bills decode to one formula: cost = kWh × tariff. A 1,500 W heater running 5 h/day for 30 days uses 225 kWh; at $0.15/kWh that’s $33.75 — which explains most “why is winter’s bill double” questions better than any theory. A photo of the meter at month start turns billing disputes into subtraction.
The percentage change and reverse-percentage patterns bills love.
Stacked discounts and the “extra 20% off” reality.
Tip percentages and fair bill splitting.
Subtract year-month-day as a calendar operation, borrowing days from the previous month when needed — not 365.25 ÷ 12 approximations. The age calculator handles leap years and month lengths exactly.
Units consumed (kWh) × slab rate, where rate rises with consumption bands, plus fixed charges and duty. Reading your meter at the same date monthly keeps the slab math honest — the electricity bill calculator mirrors slab structure.
Age Calculator Date Difference Calculator Trip Fuel Cost Calculator