Setting seconds to 0 is also adding 1 to minutes in Swift

Question

I'm trying to set the seconds of a date object to 0 with the following:

if let date = value{
   var calendar = Calendar.current
    if let mdate = calendar.date(bySetting: .second, value: 0, of: date) {
        ...
}
}

However, this isn't just setting the seconds to 0, it's also adding 1 to the minutes, and I cannot understand why.

As you can see in this screenshot - I have a date object with a time value of "15:00:10", but this becomes "15:01:00" when I set the seconds to 0.

This does not seem intuitive. Do I need to subtract a minute each time? Can I be sure that this behavior is consistent?

Answer 1

That is odd, and I'm not quite sure why it increments the minute.

You can get the result you are after by fetching the year/month/day/hour/minute component values from your date and then using those to create another date:

let value: Date? = Date()
if let date = value{
    var calendar = Calendar.current
    print(date.description(with: .current))
    
    let components = calendar.dateComponents([.year, .month, .day, .hour, .minute], from: date)
    
    if let mdate = calendar.date(from: components) {
        print(mdate.description(with: .current))
    }
}

Answer 2

The misleadingly named date(bySetting:value:of:) is, effectively, a simplified rendition of nextDate(after:matching:matchingPolicy:behavior:direction:). As the docs say, if you need more control, you should use the latter.

E.g., go .backward.

if let mdate = calendar.nextDate(after: date, matching: DateComponents(second: 0), matchingPolicy: .nextTime, direction: .backward) {
    …
}

But as has been noted below, if seconds is already zero, this will get the prior date (!) with zero seconds.

---

Better might be to extract the components (sans seconds) and then build a date from that:

let components = calendar.dateComponents([.era, .year, .month, .day, .hour, .minute], from: date)
if let mdate = calendar.date(from: components) {
    …
}

Answer 3

An alternative solution if, and I assume this based on the image, the end goal is to generate a formatted string of the time with seconds set to zero then it can be directly solved with the date formatter by replacing ss in the format with 00.

let dateformatter = DateFormatter()
dateformatter.dateFormat = "HH:mm:00"