Date and time operations

Today I explored common operations related to date and time values. In order to use the Date struct and its associated functions, we need to import the Foundation framework, which is a part of the Swift standard library.

import Foundation

Originating from early NeXTStep days, Foundation carries a lot of legacy of the early Apple operating systems. Most notably, the library includes many data types and classes prefixed with NS, such as NSDate, the NS standing for NeXTStep.

The simplest thing we may want to do with a date is to instantiate a new Date with the current date and time:

// current time in UTC
let now = Date()

If you need an epoch time (Unix timestamp), the struct has a property for that. The property is of type TimeInterval, but it can be cast as Int:

let unix = Int(now.timeIntervalSince1970)

To do the opposite, i. e. to get a Date from a Unix timestamp:

// TimeInterval seems to be an alias for Double
let someUnixTimestamp: TimeInterval = 1401667200

// Date from Unix time
let dateFromUnix = Date(timeIntervalSince1970: someUnixTimestamp)

print("Swift was first announced on \(dateFromUnix)")
// Swift was first announced on 2014-06-02 00:00:00 +0000

To add a time interval to the current timestamp, (e. g. calculate an hour from now):

// an hour from now
let hourFromNow = Date(timeIntervalSinceNow: 3600)

There is no separate initializer for doing the reverse, i. e. calculating an hour ago, but you can pass a negative value to achieve the same result:

// an hour ago
let hourAgo = Date(timeIntervalSinceNow:  -3600)

To format a timestamp as ISO 8601 time (the format used in JSON serialization):

// format a Date as ISO 8601
let nowAsISOTime = ISO8601DateFormatter().string(from: now)

To parse an ISO string as timestamp:

// parsing an ISO 8601 string as date
let isoString = "2010-04-10T21:37:00Z"

// Wrapping this expression in optional binding since it is an Optional
if let parsed = ISO8601DateFormatter().date(from: isoString) {
    print(parsed)
}

Optionals and optional bindings

Note the use of optional bindings in the last snippet. The syntax is almost identical to Rust’s, (in fact, the syntax in Rust has been borrowed from Swift), except that Rust’s optionals are enums of type Option<T> and when pattern matching, you need to match with if let Some(value) = ...:

let my_vec: Vec<Option<i32>> = vec![Some(2137), None];

for value in my_vec {
    // Here I'm also shadowing the variable `value`
    if let Some(value) = value {
        println!("Encountered an Int: {}", value);
    } else {
        println!("No value here");
    }
}

$ rustc optional.rs                   1
$ ./optional
Encountered an Int: 2137
No value here

The equivalent in Swift would be:

let my_vec: [Int?] = [2137, nil]

for value in my_vec {
    // Shadowing local variables is not permitted in Swift
    if let val = value {
        print("Encountered an Int: \(val)")
    } else {
        print("No value here")
    }
}

$ swift optional.swift
Encountered an Int: 2137
No value here

In Rust, there is an Option<T> type for nullable values and a Result<T, E> type for the results of operations that may fail. The Optional types in Swift seem to be used for both.

Formatting dates and times

Another extremely common use case is formatting dates and times as human-readable strings (like the date and time written on the lock screen of your iPhone). Foundation comes with a DateFormatter struct that does this and is highly configurable.

let dateFormatter = DateFormatter()

DateFormatter instances have the properties dateStyle and timeStyle, both of which default to DateFormatter.Style.none. Before you can format any string, you need to set a format for either, otherwise you will end up with an empty string:

print(dateFormatter.string(from: now))
// returns empty string

The possible values of DateFormatter.Style are none, short, medium, long, and full. If you want only the date or the time part, set either property to .none.

dateFormatter.dateStyle = DateFormatter.Style.medium
// since the properties are statically typed, we can omit the enum name
dateFormatter.timeStyle = .medium
// May 21, 2022, 6:03:31 PM

dateFormatter.dateStyle = .long
dateFormatter.timeStyle = .long
// May 21, 2022 at 6:03:31 PM GMT+2

dateFormatter.dateStyle = .full
dateFormatter.timeStyle = .full
// Saturday, May 21, 2022 at 6:03:31 PM Central European Summer Time

The formatters support a wide range of locales.

// Set a custom locale
dateFormatter.locale = Locale(identifier: "zh_TW")
print(dateFormatter.string(from: now))
// 2022年5月21日 星期六 下午6:09:51 [中歐夏令時間]

dateFormatter.locale = Locale(identifier: "zh_CN")
print(dateFormatter.string(from: now))
// 2022年5月21日星期六 中欧夏令时间 下午6:09:51

dateFormatter.locale = Locale(identifier: "pl_PL")
print(dateFormatter.string(from: now))
// sobota, 21 maja 2022 18:09:51 czas środkowoeuropejski letni