Get the number of days between two dates in Go

Recently I needed to find the number of days between two dates in Go and kept coming across implementations that didn't quite suit my needs.

So I rolled my own, and I'm pretty proud of how robust it is! :)

package main

import (
    "fmt"
    "time"
)

func main() {
    fmt.Println(daysBetween(date("2012-01-01"), date("2012-01-07"))) // 6
    fmt.Println(daysBetween(date("2016-01-01"), date("2017-01-01"))) // 366 because leap year
    fmt.Println(daysBetween(date("2017-01-01"), date("2018-01-01"))) // 365
    fmt.Println(daysBetween(date("2016-01-01"), date("2016-01-01"))) // 0
}

func daysBetween(a, b time.Time) int {
    if a.After(b) {
        a, b = b, a
    }

    days := -a.YearDay()
    for year := a.Year(); year < b.Year(); year++ {
        days += time.Date(year, time.December, 31, 0, 0, 0, 0, time.UTC).YearDay()
    }
    days += b.YearDay()

    return days
}

func date(s string) time.Time {
    d, _ := time.Parse("2006-01-02", s)
    return d
}

Comments

[sergi]

func daysBetween(a, b time.Time) int {
    if a.After(b) {
        a, b = b, a
    }
    return int(b.Sub(a).Hours() / 24)
}

[Sam Rose]

Not quite as robust as I'd like. Not all days have 24 hours in them. 😔

[Alexys Lozada]

func daysBetween(a, b time.Time) float64 {
    if a.After(b) {
        a, b = b, a
    }
    return math.Ceil(b.Sub(a).Hours() / 24.0)
}

[Jon Stødle]

This still doesn't work: Not all days have 24 hours.

In most countries in Europe and North America it varies between 23 and 25.

centralEuropeTime, _ := time.LoadLocation("Europe/Oslo")
twentyThreeHours := time.Date(2022, 3, 27, 0, 0, 0, 0, centralEuropeTime).Sub(time.Date(2022, 3, 28, 0, 0, 0, 0, centralEuropeTime))
twentyFiveHours := time.Date(2022, 10, 30, 0, 0, 0, 0, centralEuropeTime).Sub(time.Date(2022, 10, 31, 0, 0, 0, 0, centralEuropeTime))

[Adam Jack]

Thanks for sharing this. I like your approach. What are your thoughts on ensuring that "a" and "b" are within the same time.Location (Timezone) as each other?

For my thinking if they are in different time zones they might be viewed as in a different YearDay than accurate for the two.

I passed in time.Location, used that instead of time.UTC (although that seems moot, except for the benefits of the simplicity of consistency) and ensured that "a" and "b" are in the same TZ.

[Sam Rose]

That sounds extremely reasonable. 🙂