date diff in asp.net

Question

Hi

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

Thanks

Answer 1

If you can't use a DateTime, what do you have? Strings? In that case, use one of the DateTime.Parse() variants^* or Convert.ToDateTime() and do the comparison there. That's the only correct way. Anything else and we'll need a little more info.

* DateTime.Parse(), DateTime.TryParse(), DateTime.ParseExact(), or DateTime.TryParseExact().

Answer 2

Based on the comment, I choose that the date-time be stored as number of clock cycles since Jan 1, 1901. Now, it's a matter of simply subtracting one from the other, dividing by clock cycles per second, and further dividing by the coefficient appropriate to the unit of time you want measured (60 for minutes, 3600 for hours, 3600 * 24 for days, etc.)

Answer 3

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;
    }
}

Answer 4

I'll bet the person interviewing you was a VB dev, and expected you to answer with use the builtin VB datediff function (Microsoft.VisualBasic.DateAndTime.DateDiff). I'm not 100% but I believe that function has an overload for string or object date arguments.