List the number of days in current date/month

How to list the number of the days in the current date/month? Exp: January have 31 day so 1 2 3 4 till 31

Keep things simple.

The Go time package normalizes dates:

> The month, day, hour, min, sec, and nsec values may be outside their usual ranges and will be normalized during the conversion. For example, October 32 converts to November 1.

For the number of days in a month

func DaysInMonth(t time.Time) int {
	y, m, _ := t.Date()
	return time.Date(y, m+1, 0, 0, 0, 0, 0, time.UTC).Day()
}

For a list of days in a month

func ListDaysInMonth(t time.Time) []int {
    days := make([]int, DaysInMonth(t))
    for i := range days {
	    days[i] = i + 1
    }
    return days
}

The days in a month always range from 1 to the number of days in the given month. So the main task is to determine the number of days in a given month.

The time package does not expose such functionality, but you may use the following trick:

// Max days in year y1, month M1
t := time.Date(y1, M1, 32, 0, 0, 0, 0, time.UTC)
daysInMonth := 32 - t.Day()

The logic behind this is that the day 32 is bigger than the max day in any month. It will get automatically normalized (extra days rolled to the next month and day decremented properly). And when we subtract day we have after normalization from 32, we get exactly what the last day was in the month.

This snippet is taken from the answer https://stackoverflow.com/questions/36530251/time-since-with-months-and-years/36531443#36531443.

So here's a little helper that returns the days of a month as []int for a given time.Time:

func daysInMonth(t time.Time) []int {
    t = time.Date(t.Year(), t.Month(), 32, 0, 0, 0, 0, time.UTC)
    daysInMonth := 32 - t.Day()
    days := make([]int, daysInMonth)
    for i := range days {
        days[i] = i + 1
    }
    return days
}

Testing it:

fmt.Println(daysInMonth(time.Date(2022, 1, 1, 0, 0, 0, 0, time.UTC)))
fmt.Println(daysInMonth(time.Date(2022, 2, 1, 0, 0, 0, 0, time.UTC)))
fmt.Println(daysInMonth(time.Date(2020, 2, 1, 0, 0, 0, 0, time.UTC)))

Output (try it on the Go Playground):

[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31]
[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28]
[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29]

Another, less peformant option is to roll back the date to the first of the month, and start adding days until the month changes. This is how it could look like:

func daysInMonth(t time.Time) []int {
	var days []int

	// Roll back to day 1
	t = time.Date(t.Year(), t.Month(), 1, 0, 0, 0, 0, time.UTC)
	m := t.Month()
	for t.Month() == m {
		days = append(days, t.Day())
		t = t.AddDate(0, 0, 1)
	}

	return days
}

This will output the same. Try this one on the Go Playground.

Since all months contain at least 28 days, we can optimize the above solution to roll to day 29, and start checking and incrementing from there:

func daysInMonth(t time.Time) []int {
	days := make([]int, 28, 31)
	for i := range days {
		days[i] = i + 1
	}

	m := t.Month()
	// Roll to day 29
	t = time.Date(t.Year(), t.Month(), 29, 0, 0, 0, 0, time.UTC)
	for t.Month() == m {
		days = append(days, t.Day())
		t = t.AddDate(0, 0, 1)
	}

	return days
}

Try this one on the Go Playground.

There are many ways to get it done, as we are dealing with constants mostly, and we just need to handle February if it's a Leap Year( 29 days).

package main
import (
"fmt"
"time"
)
func main() {
fmt.Println(list_of_days())
}
func list_of_days() []int {
non_leap_year := map[string]int{
"January":   31,
"February":  28,
"March":     31,
"April":     30,
"May":       31,
"June":      30,
"July":      31,
"August":    31,
"September": 30,
"October":   31,
"November":  30,
"December":  31,
}
leap_year := map[string]int{
"January":   31,
"February":  29,
"March":     31,
"April":     30,
"May":       31,
"June":      30,
"July":      31,
"August":    31,
"September": 30,
"October":   31,
"November":  30,
"December":  31,
}
//get the current month
year, month, _ := time.Now().Date()
//handle leap year
no_of_days := 0
if year%4 == 0 && year%100 != 0 || year%400 == 0 {
no_of_days = leap_year[month.String()]
} else {
no_of_days = non_leap_year[month.String()]
}
days := make([]int, no_of_days)
for i := range days {
days[i] = i + 1
}
return days
}