Question

I was trying to save some stuff to the Log table with timestamp so I first did this:

public static string TimeStamp(
   this DateTime datetime, string timestamptFormat = "yyyyMMddHHmmssffff")
   {
     return datetime.ToString(timestamptFormat);
   }

And then I found a snippet like this:

static public string ToReverseTimestamp(this DateTime dateTime)
{
return string.Format("{0:10}", DateTime.MaxValue.Ticks - dateTime.Ticks);
}

I started wondering what the heck is reverse timestamp is useful for, and came across this article

Now my question is: if the second snippet is even correct? And how do you convert it back to “normal” timestamp or how do you get readable datetime information from it?

Answer 1

Make sure that the DateTime is converted to universal time before conversion to avoid time-zone problems:

public static string ToReverseTimestamp(this DateTime dateTime)
{
    return (long.MaxValue - dateTime.ToUniversalTime().Ticks).ToString();
}

You can convert the value back to a DateTime value by parsing the string to a long, calculating MaxValue - (MaxValue - x) = x and constructing a new DateTime with DateTimeKind.Utc from x:

public static DateTime FromReverseTimestamp(string timestamp)
{
    return new DateTime(long.MaxValue - long.Parse(timestamp), DateTimeKind.Utc);
}

Example:

var input = DateTime.Now;                      // {17/05/2012 16:03:17} (Local)
var timestamp = ToReverseTimestamp(input);     // "2520650302020786038"
var result = FromReverseTimestamp(timestamp);  // {17/05/2012 18:03:17} (Utc)