What is the formula for calculating a timestamp?

Question

Just a random question but does anyone know the formula for how a timestamp is calculated? I am guessing it has to consider each month and how many days, leap years, etc.

Thanks

Answer 1

Usually it is the elapsed time since a specific date. In the case of unix time, it is the time elapsed since Jan 1, 1970 in seconds.

Answer 2

Here's an example of how Unix timestamp is calculated from the wikipedia article:

The Unix time number is zero at the Unix epoch, and increases by exactly 86 400 per day since the epoch. Thus 2004-09-16T00:00:00Z, 12 677 days after the epoch, is represented by the Unix time number 12 677 × 86 400 = 1 095 292 800. This can be extended backwards from the epoch too, using negative numbers; thus 1957-10-04T00:00:00Z, 4 472 days before the epoch, is represented by the Unix time number −4 472 × 86 400 = -386 380 800.

Answer 3

If you are interested in implementation here is roughly how Windows timestamp can be calculated (also known as ticks):

public static Int64 GetTimeStamp(
                        int year, int month, int day,
                        int hour, int minute, int second, int milliseconds)
{
    Int64 timestamp = DateToTicks(year, month, day)
        + TimeToTicks(hour, minute, second);

    return timestamp + milliseconds * TicksInMillisecond;
}

static readonly int[] DaysToMonth365 =
    new int[] { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 };
static readonly int[] DaysToMonth366 =
    new int[] { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 };
const long TicksInSecond = TicksInMillisecond * 1000L;
const long TicksInMillisecond = 10000L;

public static bool IsLeapYear(int year)
{
    if ((year < 1) || (year > 9999))
        throw new ArgumentOutOfRangeException("year", "Bad year.");

    if ((year % 4) != 0)
        return false;

    if ((year % 100) == 0)
        return ((year % 400) == 0);

    return true;
}

private static long DateToTicks(int year, int month, int day)
{
    if (((year >= 1) && (year <= 9999)) && ((month >= 1) && (month <= 12)))
    {
        int[] daysToMonth = IsLeapYear(year) ? DaysToMonth366 : DaysToMonth365;
        if ((day >= 1) && (day <= (daysToMonth[month] - daysToMonth[month - 1])))
        {
            int previousYear = year - 1;
            int daysInPreviousYears = ((((previousYear * 365) + (previousYear / 4)) - (previousYear / 100)) + (previousYear / 400));

            int totalDays = ((daysInPreviousYears + daysToMonth[month - 1]) + day) - 1;
            return (totalDays * 0xc92a69c000L);
        }
    }
    throw new ArgumentOutOfRangeException();
}

private static long TimeToTicks(int hour, int minute, int second)
{
    long totalSeconds = ((hour * 3600L) + (minute * 60L)) + second;
    if ((totalSeconds > 0xd6bf94d5e5L) || (totalSeconds < -922337203685L))
        throw new ArgumentOutOfRangeException();

    return (totalSeconds * TicksInSecond);
}