c# - Converting date to DOS date -

i having trouble dos date formats. need convert:

 from:        29th june of 2011  to:        16093 

i know 16093 correct result, how can result?

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i can convert dos date integer value recognized datetime don't know how reverse process. how conversion dos date datetime:

var number = 16093;  var year = (number >> 9) + 1980; var month = (number & 0x01e0) >> 5; var day = number & 0x1f; var date = new datetime((int)year, (int)month, (int)day); 

this works. need reverse it.

let me teach how fish, , in process fix problem.

1. what have?

   a datetime .net object represents date/time combination.

   datetime datetime = new datetime(2011, 6, 29); 

2. what want?

   a dos date/time according specification, 16-bit date component. time component ignored.

3. what need?

   the day of month, month number, , year since 1980. let's them unsigned integers , ensure in proper range.

   uint day = (uint)datetime.day;              // between 1 , 31 uint month = (uint)datetime.month;          // between 1 , 12 uint years = (uint)(datetime.year - 1980);  // 1980 

   note there 7 bits year, can represent years 1980 2107. outside of range invalid. since years unsigned, can't represent negative values. value 1970 (minus 1980) 4294967286, out of range.

   if (years > 127)     throw new argumentoutofrangeexception("cannot represent year."); 

4. how combine it?

   we need shift values appropriate spot in resulting integer.

   start all-zero integer. need lower 16 bits convenience we'll use uint here.

   uint dosdatetime = 0; 

   the specification lists bit indices of each value starts. since we're ignoring time part (the 16 least significant bits), need subtract 16 values.

   days start @ bit 16. shift left 16 - 16 positions (i.e. no shifting), , can or bits result.

   dosdatetime |= day << (16 - 16); 

   months start @ bit 21 (minus 16), , years @ bit 25.

   dosdatetime |= minutes << (21 - 16); dosdatetime |= years << (25 - 16); 

   then turn unsigned 16-bit integer. won't overflow.

   ushort result = unchecked((ushort)dosdatetime); 

5. how put together?

   putting extension method:

   public static class datetimeextensions {     public static ushort todosdatetime(this datetime datetime)     {         uint day = (uint)datetime.day;              // between 1 , 31         uint month = (uint)datetime.month;          // between 1 , 12         uint years = (uint)(datetime.year - 1980);  // 1980          if (years > 127)             throw new argumentoutofrangeexception("cannot represent year.");          uint dosdatetime = 0;         dosdatetime |= day << (16 - 16);         dosdatetime |= month << (21 - 16);         dosdatetime |= years << (25 - 16);          return unchecked((ushort)dosdatetime);     } } 

   of course can write more concise, shows what's going on. , compiler optimize many of constants.

6. testing it

   you should write unit tests see code works correctly. quick test verifies basic idea:

   static void main(string[] args) {     datetime datetime = new datetime(2011, 6, 29);     ushort dosdatetime = datetime.todosdatetime();     console.writeline(dosdatetime);     // prints: 16093     console.readline(); }