Calculate Days Between Dates

daysdiff.go

// original post: https://groups.google.com/d/msg/golang-nuts/O2NaRAH94GI/eXrvZtsTCQAJ
package days

import (
	"fmt"
	"time"
)

func lastDayOfYear(t time.Time) time.Time {
	return time.Date(t.Year(), 12, 31, 0, 0, 0, 0, t.Location())
}

func firstDayOfNextYear(t time.Time) time.Time {
	return time.Date(t.Year()+1, 1, 1, 0, 0, 0, 0, t.Location())
}

// a - b in days
func daysDiff(a, b time.Time) (days int) {
	cur := b
	for cur.Year() < a.Year() {
		// add 1 to count the last day of the year too.
		days += lastDayOfYear(cur).YearDay() - cur.YearDay() + 1
		cur = firstDayOfNextYear(cur)
	}
	days += a.YearDay() - cur.YearDay()
	if b.AddDate(0, 0, days).After(a) {
		days -= 1
	}
	return days
}