Calculate "Start" Time from "End" Time

Question

I'm working to create a function that takes an input duration and an "end" time, and calculates a "start" time based on that criteria.

Example: if the "end" time is 5:00pm and the "duration" is 30 minutes, then the "start" time returned should be 4:30pm.

I'm getting lost in translating between Int/Date/String, so I may need to re-think my entire approach. So far I'm working with the following, which is just a take on this Stack Overflow post:

var userInputEventEndTime: String
var userInputEventDuration: Int
var userInputEventNeedsToStartTime: String

//Manipulate this function with the input parameters
 func makeDate(hr: Int, min: Int, sec: Int) -> Date {
    var calendar = Calendar(identifier: .gregorian)
    let components = DateComponents(hour: hr, minute: min, second: sec)
    let hour = calendar.component(.hour, from: calendar.date(from: components)!)
    let minutes = calendar.component(.minute, from: calendar.date(from: components)!)
    let seconds = calendar.component(.second, from: calendar.date(from: components)!)
    return calendar.date(from: components)!
}

Should I be converting the input strings to DateComponents and then doing the math? If so, I'm not sure how to account for minutes converting to hours. Or just go a different direction altogether? Thanks!

Is your event only given as a time without a date part? – Joakim Danielson Nov 01 '19 at 16:02 — Joakim Danielson, Nov 01 '19 at 16:02
Correct; no year/month/day context necessary. – moosgrn Nov 01 '19 at 16:03 — moosgrn, Nov 01 '19 at 16:03

Joakim Danielson · Accepted Answer · 2019-11-01T18:19:59.960

The following uses DateComponents and Calendar to calculate the start time

let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "hh:mm a"
let calendar = Calendar.current
let dateComponents = DateComponents(calendar: calendar, minute: -userInputEventDuration)

if let endTime = dateFormatter.date(from: userInputEventEndTime),
    let startTime = calendar.date(byAdding: dateComponents, to: endTime, wrappingComponents: false) {
    userInputEventNeedsToStartTime = dateFormatter.string(from: startTime)
}

Here is the same solution written as a function with an example

func calculateStartTime(from endTime: String, duration: Int) -> String? {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "hh:mm a"
    let calendar = Calendar.current

    let dateComponents = DateComponents(calendar: calendar, minute: -duration)
    if let time = dateFormatter.date(from: endTime),
        let startTime = calendar.date(byAdding: dateComponents, to: time, wrappingComponents: false) {
        return dateFormatter.string(from: startTime)
    }

    return nil
}

if let startTime = calculateStartTime(from: "6:00 PM", duration: 30) {
    print("Event starts at \(startTime)")
}

Output

Event starts at 05:30 PM

score 0 · Answer 2 · answered Nov 01 '19 at 15:51

0

To subtract components from the date, just add their negative values.

answered Nov 01 '19 at 15:51

kkiermasz

464
2
15

Calculate "Start" Time from "End" Time

2 Answers2