Date arithmetic is easy to get subtly wrong — month lengths, leap years and weekends all get in the way. The calculators here answer the common questions quickly: how many days (and business days) lie between two dates, what date is 90 days from now, and which weekday a given date falls on.
Calculations run locally in your browser using the date-fns library. For Unix timestamps and timezone conversion, see the Epoch & Timezone tools.