Calculate the number of days between two dates

Count whole calendar days between two YYYY-MM-DD dates.

JavaScript Version 1.0.0 · MIT · .md

function diffDays({ first, second }) { return Math.round(Math.abs(Date.parse(second) - Date.parse(first)) / 86400000); }

Input & expected output

{
  "first": "2014-12-19",
  "second": "2020-01-01"
}

1839

The edge case

{
  "first": "2026-10-28",
  "second": "2026-10-28"
}

0

Know the limits

• Both dates are UTC calendar days, so daylight-saving changes cannot add or lose an hour. The original rounded local Date differences up with Math.ceil.
• The result is absolute: the order of the dates does not change it.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Compare two dates — Compare validated ISO calendar dates in UTC and return -1, 0 or 1.
• Add AM PM suffix to an hour — Turn an hour from 0 to 23 into a 12-hour label with am or pm.
• Convert a date to YYYY-MM-DD format — Format an ISO date-time as its UTC calendar date.