DateTime conversion

Balazs Farkas

Hello Guys,

I've a file which is in some binary format I don't know, but I need to be able to read/write it. It contains data for given dates. I'm able to get the data part (some numbers for each days), but I couldn't figure out the dates.

Example (as it is in the file):

HEX, Decimal, Date Value (what it represents)
-----------------------
[66 5f 7f ff] (102,95,127,255) - (2002.05.30)
[66 5f ff ff] (102,95,255,255) - (2002.05.31)

So I need to know how I get the date values from the numbers.

I think it is written in reverse. I tried to convert it to Int32 and check the differences:

255,127,95,102 = 1717534719
255,255,95,102 = 1717567487

The difference is 32768, so that would make 1 day..., but unfortunately it's not always true (ie. I tested a year end example and it was not the case).

So, I would like to ask your help in figuring this out. How to convert it to DateTime and vice-verse.

Thanks,
Balazs

6 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes15y ago
Accepted answer
OK, this code should do it:

using System;

class DateConverter
{
static void Main()
{
byte[] bytes1 = {0x66, 0x5f, 0x7f, 0xff};
DateTime date1 = ToDateTime(bytes1);
Console.WriteLine(date1.ToLongDateString());

byte[] bytes2 = {0x66, 0x5f, 0xff, 0xff};
DateTime date2 = ToDateTime(bytes2);
Console.WriteLine(date2.ToLongDateString());

byte[] bytes3 = {0x64, 0x11, 0xff, 0xff};
DateTime date3 = ToDateTime(bytes3);
Console.WriteLine(date3.ToLongDateString());

byte[] bytes4 = {0x65, 0x11, 0xff, 0xff};
DateTime date4 = ToDateTime(bytes4);
Console.WriteLine(date4.ToLongDateString());

Console.WriteLine();

DateTime date5 = new DateTime(2001, 1, 4);
byte[] bytes5 = FromDateTime(date5);
PrintBytes(bytes5);

DateTime date6 = new DateTime(2001, 1, 5);
byte[] bytes6 = FromDateTime(date6);
PrintBytes(bytes6);

DateTime date7 = new DateTime(2001, 1, 6);
byte[] bytes7 = FromDateTime(date7);
PrintBytes(bytes7);

DateTime date8 = new DateTime(2001, 1, 7);
byte[] bytes8 = FromDateTime(date8);
PrintBytes(bytes8);

Console.WriteLine();

// convert first 4 in opposite direction

bytes1 = FromDateTime(date1);
PrintBytes(bytes1);

bytes2 = FromDateTime(date2);
PrintBytes(bytes2);

bytes3 = FromDateTime(date3);
PrintBytes(bytes3);

bytes4 = FromDateTime(date4);
PrintBytes(bytes4);

Console.ReadKey();

}

static DateTime ToDateTime(byte[] bytes)
{
int year = bytes[0] + 1900;

// get first 4 bits of 2nd byte
int month = (bytes[1] & 0xf0) >> 4;

// get last 4 bits of 2nd byte
int temp = bytes[1] & 0x0f;

// get first bit of 3rd byte
int temp2 = (bytes[2] & 0x80) >> 7;

int day = temp * 2 + temp2;

return new DateTime(year, month, day);
}

static byte[] FromDateTime(DateTime dt)
{
byte[] bytes = new byte[4];
bytes[3] = 0xff; // always 255
bytes[0] = (byte)(dt.Year - 1900);
int month = dt.Month;
int day = dt.Day;

// get first 4 bits of day
int temp = (day & 0x1e) >> 1;

// get last bit of day
int temp2 = day & 0x01;

bytes[1] = (byte)(month * 16 + temp);
bytes[2] = (byte)(temp2 * 128 + 127);
return bytes;
}

static void PrintBytes(byte[] bytes)
{
Console.Write("[");
for(int i = 0; i < bytes.Length; i++)
{
Console.Write("{0:x2}", bytes[i]);
if (i < bytes.Length - 1) Console.Write(" ");
}
Console.WriteLine("]");
}
}

The output should be:

30 May 2002
31 May 2002
03 January 2000
03 January 2001

[65 12 7f ff]
[65 12 ff ff]
[65 13 7f ff]
[65 13 ff ff]

[66 5f 7f ff]
[66 5f ff ff]
[64 11 ff ff]
[65 11 ff ff]
Balazs Farkas15y ago
Thanks, it's exactly what I needed :)
Balazs Farkas15y ago
Sure, I would be happy to have some conversion routine. :)

Thanks.
Vulpes15y ago
Yeah, I think I've figured it out now :)

Of the remaining 3 bytes (24 bits):

The first 4 bits represents the month from 1 to 12;

The next 5 bits represents the day from 1 to 31;

The remaining 15 bits are unused and are all set to 1.

So 30th May would be encoded as:

01011111 01111111 11111111

or in hex : 5f 7f ff

31st May would be:

01011111 11111111 11111111

or in hex: 5f ff ff

and 3rd January would be:

00010001 11111111 11111111

or in hex: 11 ff ff

If you can give me a little time, I'll try and come up with some conversion routines to and from the .NET DateTime type.
Balazs Farkas15y ago
[64 11 ff ff] (100,17,255,255) - 2000.01.03 (3rd of january)
[65 11 ff ff] (101,17,255,255) - 2001.01.03
[65 12 7f ff] (101,18,127,255) - 2001.01.04
[65 12 ff ff] (101,18,255,255) - 2001.01.05
[65 13 7f ff] (101,19,127,255) - 2001.01.06
[65 13 ff ff] (101,19,255,255) - 2001.01.07

It looks like you were right. First one is the year since 1900. I think than it increasing the second one by 1 for every second day and flipping the third one between 255 and 127. What do you think?

It's just hard to believe that it's not some date format but some kind of code.
Vulpes15y ago
Although there are some 4 byte 'packed decimal' formats for dates, I don't recognize this one :-/

Do you have the representations for a few other dates which might help us to figure out what it is.

A tentative theory is that 102 might be the number of years since 1900.