Get list of time.Time based on nth day of month logic

ScheduleNthWeekdayOfMonth.go

package main

import (
	"fmt"
	"time"
)

func main() {

	// TODO
	//  Add error checking and validation across the board (e.g., fifth occurrence of weekday).
	//  Consider implementing a ScheduleLastWeekdayOfMonth function, or...
	//  Consider assuming last weekday of month if user supplied time is the last week of the month.

	// User supplies start date from timepicker.
	// User supplies end time from number of years to run or from timepicker.
	// Generate a slice of time.Time for the scheduler.
	start := time.Date(2022, 1, 11, 14, 0, 0, 0, time.UTC) // Second Tuesday of every month at 14:00.
	end := start.AddDate(3, 0, 0)                          // Run for 3 years.
	schedule := ScheduleNthWeekdayOfMonth(start, end)

	// Creating run once style jobs based on generated schedule.
	s := gocron.NewScheduler(time.UTC)
	for _, t := range schedule {
		_, err := s.Every(1).Tag("some-guid").LimitRunsTo(1).StartAt(t).Do(func() {})
		if err != nil {
			log.Fatal(err)
		}
	}

	s.StartBlocking()
}

// ScheduleNthWeekdayOfMonth returns a slice of monthly time.Time based on nth weekday of the month logic.
// Day of week and week of month are derived from the supplied start time.
// Monthly time.Time structs are generated until end time is reached.
func ScheduleNthWeekdayOfMonth(start time.Time, end time.Time) []time.Time {

	var schedule []time.Time

	// Set initial day of week.
	dayOfWeek := int(start.Weekday())

	// Derive the nth occurrence of the weekday in the month from the supplied start time.
	weekOfMonth := getNthOccurrenceOfWeekday(start)

	// Increment start time by one month until end time is reached.
	// Generate time struct for each month based on nth weekday of month logic.
	for start.Before(end) {
		dayOfMonth := getNthWeekdayOfMonth(start.Year(), int(start.Month()), dayOfWeek, weekOfMonth)
		schedule = append(schedule, time.Date(start.Year(), start.Month(), dayOfMonth, start.Hour(), start.Minute(), start.Second(), 0, start.Location()))
		start = start.AddDate(0, 1, 0)
	}

	return schedule
}

// getNthOccurrenceOfWeekday returns the nth occurrence of the weekday based on the provided time struct.
// Iterates over getNthWeekdayOfMonth until supplied day matches.
// Returns -1 if supplied date occurs in fifth week or no match occurs.
func getNthOccurrenceOfWeekday(t time.Time) int {
	for i := 0; i < 4; i++ {
		if t.Day() == getNthWeekdayOfMonth(t.Year(), int(t.Month()), int(t.Weekday()), i) {
			return i
		}
	}
	return -1
}

// getNthWeekdayOfMonth returns the day of the month based on nth weekday of the month logic.
// Takes the target year and month along with the weekday and desired week.
// First week of the month is 0.
func getNthWeekdayOfMonth(year, month, weekday, week int) int {

	// Create time struct for the first day of the month.
	t := time.Date(year, time.Month(month), 1, 0, 0, 0, 0, time.UTC)

	// Step from the first of the month to the target day.
	t = t.AddDate(0, 0, -(int(t.Weekday()) - weekday))

	// If the step to the target day goes to the prior month, add 1 week to the multiplier
	if t.Day() > 7 {
		week++
	}

	// Add the requisite number of weeks and return the day of the month.
	return t.AddDate(0, 0, week*7).Day()
}