Question

Can anyone tell me how to calculate the diff between two dates without using .net datetime and timestamp classes?

Thanks

Answer 1

If I couldn’t use the built-in types, I’d try to find another .NET date/time API. I suspect I wouldn’t find one though, given that any sane person would use the built-in ones.

If it’s really just a case of dates, I guess a full date/time API isn’t really required. Parse whatever you get into “days since some epoch” (e.g. Jan 1st 1970) and just do normal subtraction.

Be careful of leap years though – and things get even more fun if you need to cope with the bizarre calendar changes a few centuries back.

Sample code assuming a format of “yyyyMMdd”, not doing any error checking, not coping with dates before 1900, and not worrying about efficiency at all:

using System;

struct Date
{
    readonly int year;
    readonly int month;
    readonly int day;

    public Date(int year, int month, int day)
    {
        this.year = year;
        this.month = month;
        this.day = day;
    }

    public static Date Parse(string text)
    {
        return new Date(int.Parse(text.Substring(0, 4)),
                        int.Parse(text.Substring(4, 2)),
                        int.Parse(text.Substring(6, 2)));
    }

    // Days since first Jan 1st 1900
    public int DaysSinceEpoch
    {
        get
        {
            int days = 0;
            for (int i = 1900; i < year; i++)
            {
                days += IsLeapYear(i) ? 366 : 365;
            }
            for (int i = 1; i < month; i++)
            {
                days += GetMonthLength(i, year);
            }
            days += day - 1;
            return days;
        }
    }

    private static readonly int[] MonthLengths = 
    { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    private static int GetMonthLength(int month, int year)
    {
        return MonthLengths[month-1] + 
            ((IsLeapYear(year) && month == 2) ? 1 : 0);
    }

    private static bool IsLeapYear(int year)
    {
        // http://en.wikipedia.org/wiki/Leap_year
        if ((year % 4) != 0) return false;
        if ((year % 400) == 0) return true;
        if ((year % 100) == 0) return false;
        return true;
    }
}

class Test
{
    static void Main(string[] args)
    {
        Console.WriteLine(DateDiff("19040301", "19050301")); // 365
        Console.WriteLine(DateDiff("19040201", "19050201")); // 366
        Console.WriteLine(DateDiff("19760619", "20090529")); // I feel old
    }

    static int DateDiff(string first, string second)
    {
        Date firstDate = Date.Parse(first);
        Date secondDate = Date.Parse(second);
        return secondDate.DaysSinceEpoch - firstDate.DaysSinceEpoch;
    }
}