Calculate number of weeks in a given month - Swift - swift

Question

Having looked at a few different suggestions on SO, I've not been able to determine why the function below does not work. It seems to return 6 for some months and 5 for others. When changing weeks for days it works perfectly.
For example, trying
weeksInMonth(8, forYear 2015)
Results in 6.
I believe I have a mis-understanding of what 'firstWeekday' property on calendar does but haven't found an adequate explanation by Apple or online.
Have tried both .WeekOfMonth and .WeekOfYear. Again can't find explanation of exact difference.
Any suggestions would be greatly welcome.
func weeksInMonth(month: Int, forYear year: Int) -> Int?
{
if (month < 1 || month > 12) { return nil }
let dateString = String(format: "%4d/%d/01", year, month)
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy/MM/dd"
if let date = dateFormatter.dateFromString(dateString),
let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
{
calendar.firstWeekday = 2 // Monday
let weekRange = calendar.rangeOfUnit(.WeekOfMonth, inUnit: .Month, forDate: date)
let weeksCount = weekRange.length
return weeksCount
}
else
{
return nil
}
}
Update:
Apologies my question was not clear. I'm trying to work out how many weeks there are in a month that include a Monday in them. For August this should be 5.

Answer 1

Your code computes the number of weeks which occur (complete or partially)
in a month. What you apparently want is the number of Mondays
in the given month. With NSDateComponents and in particular with
the weekdayOrdinal property you can compute the first
(weekdayOrdinal=1) and last (weekdayOrdinal=-1) Monday
in a month. Then compute the difference in weeks (and add one).
A possible implementation in Swift 2:
func numberOfMondaysInMonth(month: Int, forYear year: Int) -> Int?
{
let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
calendar.firstWeekday = 2 // 2 == Monday
// First monday in month:
let comps = NSDateComponents()
comps.month = month
comps.year = year
comps.weekday = calendar.firstWeekday
comps.weekdayOrdinal = 1
guard let first = calendar.dateFromComponents(comps) else {
return nil
}
// Last monday in month:
comps.weekdayOrdinal = -1
guard let last = calendar.dateFromComponents(comps) else {
return nil
}
// Difference in weeks:
let weeks = calendar.components(.WeekOfMonth, fromDate: first, toDate: last, options: [])
return weeks.weekOfMonth + 1
}
Note: That a negative weekdayOrdinal counts backwards from the end of the month is not apparent form the documentation. It was observed in
Determine NSDate for Memorial Day in a given year and confirmed by Dave DeLong).
Update for Swift 3:
func numberOfMondaysInMonth(_ month: Int, forYear year: Int) -> Int? {
var calendar = Calendar(identifier: .gregorian)
calendar.firstWeekday = 2 // 2 == Monday
// First monday in month:
var comps = DateComponents(year: year, month: month,
weekday: calendar.firstWeekday, weekdayOrdinal: 1)
guard let first = calendar.date(from: comps) else {
return nil
}
// Last monday in month:
comps.weekdayOrdinal = -1
guard let last = calendar.date(from: comps) else {
return nil
}
// Difference in weeks:
let weeks = calendar.dateComponents([.weekOfMonth], from: first, to: last)
return weeks.weekOfMonth! + 1
}

Answer 2

Actually your question is : How many Mondays are in a given month?
My approach is to calculate the first Monday for the month, this can be accomplished by setting the CalendarUnit WeekdayOrdinal to 1. Then get the number of total days and do some math.
Swift 1.2
func mondaysInMonth(month: Int, forYear year: Int) -> Int?
{
if 1...12 ~= month {
let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
let components = NSDateComponents()
components.weekday = 2 // Monday
components.weekdayOrdinal = 1
components.month = month
components.year = year
if let date = calendar.dateFromComponents(components) {
let firstDay = calendar.component(.CalendarUnitDay, fromDate: date)
let days = calendar.rangeOfUnit(.CalendarUnitDay, inUnit:.CalendarUnitMonth, forDate:date).length
return (days - firstDay) / 7 + 1
}
}
return nil
}
Swift 2
func mondaysInMonth(month: Int, forYear year: Int) -> Int?
{
guard 1...12 ~= month else { return nil }
let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
let components = NSDateComponents()
components.weekday = 2 // Monday
components.weekdayOrdinal = 1
components.month = month
components.year = year
if let date = calendar.dateFromComponents(components) {
let firstDay = calendar.component(.Day, fromDate: date)
let days = calendar.rangeOfUnit(.Day, inUnit:.Month, forDate:date).length
return (days - firstDay) / 7 + 1
}
return nil
}

Answer 3

Swift 2
The following code calculates the number of weeks in a month. It does not depend on what day the month started or ended.
func weeksInMonth(month: Int, year: Int) -> (Int)? {
let calendar = NSCalendar.currentCalendar()
let comps = NSDateComponents()
comps.month = month+1
comps.year = year
comps.day = 0
guard let last = calendar.dateFromComponents(comps) else {
return nil
}
// Note: Could get other options as well
let tag = calendar.components([.WeekOfMonth,.WeekOfYear,
.YearForWeekOfYear,.Weekday,.Quarter],fromDate: last)
return tag.weekOfMonth
}
// Example Usage:
if let numWeeks = weeksInMonth(8,year: 2015) {
print(numWeeks) // Prints 6
}
if let numWeeks = weeksInMonth(12,year: 2015) {
print(numWeeks) // Prints 5
}

Answer 4

Swift 4+:
//-- Get number of weeks from calendar
func numberOfWeeksInMonth(_ date: Date) -> Int {
var calendar = Calendar(identifier: .gregorian)
calendar.firstWeekday = 1
let weekRange = calendar.range(of: .weekOfMonth,
in: .month,
for: date)
return weekRange!.count
}
//-- Implementation
let weekCount = numberOfWeeksInMonth(Date)

Answer 5

Swift 4+:
Generate Date from your components:
let dateComponents = DateComponents.init(year: 2019, month: 5)
let monthCurrentDayFirst = Calendar.current.date(from: dateComponents)!
let monthNextDayFirst = Calendar.current.date(byAdding: .month, value: 1, to: monthCurrentDayFirst)!
let monthCurrentDayLast = Calendar.current.date(byAdding: .day, value: -1, to: monthNextDayFirst)!
Detect week number from date:
let numberOfWeeks = Calendar.current.component(.weekOfMonth, from: monthCurrentDayLast)