golang can calculate the difference between two dates. This includes year, months and days.

The problem can be quite hard, as you have to keep in mind years, months and days. Some years have leap years, not all months are 30 days and so on.

It will calculate the difference in years, months, days and also total number of days difference.

Date difference in golang

Example

The golang program below calculates the difference between two dates.

package main

import (
    "fmt"
    "time"
)

func main() {
        start := time.Date(1990, 2, 1, 3, 30, 0, 0, time.UTC)

    fmt.Println(start)
    fmt.Println(time.Now())
    
        // calculate years, month, days and time betwen dates
        year, month, day, hour, min, sec := diff(start, time.Now())

        fmt.Printf("difference %d years, %d months, %d days, %d hours, %d mins and %d seconds.", year, month, day, hour, min, sec)
        fmt.Printf("")

        // calculate total number of days
    duration := time.Now().Sub(start)
    fmt.Printf("difference %d days", int(duration.Hours()/24) )
}

func diff(a, b time.Time) (year, month, day, hour, min, sec int) {
    if a.Location() != b.Location() {
        b = b.In(a.Location())
    }
    if a.After(b) {
        a, b = b, a
    }
    y1, M1, d1 := a.Date()
    y2, M2, d2 := b.Date()

    h1, m1, s1 := a.Clock()
    h2, m2, s2 := b.Clock()

    year = int(y2 - y1)
    month = int(M2 - M1)
    day = int(d2 - d1)
    hour = int(h2 - h1)
    min = int(m2 - m1)
    sec = int(s2 - s1)

    // Normalize negative values
    if sec < 0 {
        sec += 60
        min--
    }
    if min < 0 {
        min += 60
        hour--
    }
    if hour < 0 {
        hour += 24
        day--
    }
    if day < 0 {
        // days in month:
        t := time.Date(y1, M1, 32, 0, 0, 0, 0, time.UTC)
        day += 32 - t.Day()
        month--
    }
    if month < 0 {
        month += 12
        year--
    }

    return
}

This will output something like

1980-03-24T00:00:00-05:00
2018-06-16T14:31:32-04:00
difference 38 years, 2 months, 23 days.
difference 13963 days.

Exercises

1. Calculate the difference between new year and now.
2. How many days until next year?