I use this to know how many hours I work a day.

TimeTracking.swift

#!/usr/bin/env xcrun swift
// Usage: path/to/TimeTracking.swift

// Go to bottom to see how it's used.

// The MIT License (MIT)

// Copyright (c) 2015 Nicholas T.

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

typealias TimeInputType = Int // hhmm

private let OnehourInMinutes = 60

struct _Time: Printable {
    let hours: Int
    let minutes: Int

    init(hours: Int, minutes: Int) {
        (self.hours, self.minutes) = (hours, minutes)
    }

    init(_ time: TimeInputType) {
        (self.hours, self.minutes) = (time/100, time % 100)
    }

    var asTimeInput: TimeInputType { return hours * 100 + minutes  }

    var description: String {
        return "\(hours)h \(minutes)m"
    }
}

func + (left: _Time, right: _Time) -> _Time {
    var sumOfMinutes = left.minutes + right.minutes
    var sumOfHours = left.hours + right.hours

    if sumOfMinutes > OnehourInMinutes {
        sumOfMinutes -= OnehourInMinutes
        sumOfHours += 1
    }

    return _Time(hours: sumOfHours, minutes: sumOfMinutes)
}
func - (left: _Time, right: _Time) -> _Time {
    var differenceOfMinutes = left.minutes - right.minutes
    var differenceOfHours = left.hours - right.hours

    if differenceOfMinutes < 0 {
        differenceOfMinutes += OnehourInMinutes
        differenceOfHours -= 1
    }

    return _Time(
        hours: differenceOfHours,
        minutes: differenceOfMinutes
    )
}

struct TimeDuration: Printable {
    let from: _Time
    let to: _Time
    let base: _Time

    init(base: _Time) {
        self.base = base
        self.from = _Time(0000)
        self.to = _Time(0000)
    }

    init(from: TimeInputType,
        to: TimeInputType,
        base: TimeInputType = TimeInputType()
        ) {
            self.from = _Time(from)
            self.to = _Time(to)
            self.base = _Time(base)
    }

    var durationAsTimeInput: TimeInputType {
        return duration.asTimeInput
    }

    var duration: _Time {
        let duration = base + to - from
        return duration
    }

    var description: String {
        return "\(duration.hours)h \(duration.minutes)m"
    }
}

func + (left: TimeDuration, right: TimeDuration) -> TimeDuration {
    return TimeDuration(base: left.duration + right.duration)
}

struct ToTime {
    let from: TimeInputType
    let base: TimeInputType
    init(
        from: TimeInputType,
        base: TimeInputType = TimeInputType()
        ) {
            self.from = from
            self.base = base
    }

    func to(time: TimeInputType) -> TimeDuration {
        return TimeDuration(from: from, to: time, base: base)
    }
}

struct Time {
    static func from(time: TimeInputType, base: TimeInputType = TimeInputType()) -> ToTime {
        return ToTime(from: time, base: base)
    }
}

extension TimeDuration {
    func from(time: TimeInputType) -> ToTime {
        return Time.from(time, base: durationAsTimeInput)
    }
}

func main() {
    let date = "2015-12-03 and 2015-12-04"
    let hoursOn3rd = Time.from(1610).to(1620)
    let hoursOn4th = Time.from(1420).to(1440)

    let hours = hoursOn3rd + hoursOn4th

    println("Date: \(date)")  // Date: 2015-12-03 and 2015-12-04
    println("Time: \(hours)") // Time: 0h 28m
}

main()