Understanding MS SQL Server Date Types Consider the following:

declare @dt datetime, @dt2 datetime2, @d date
set @dt  = '2013-01-01'
set @dt2 = '2013-01-01'
set @d   = '2013-01-01'

select convert(varbinary, @dt) as dt,
       convert(varbinary, @dt2) as dt2,
       convert(varbinary, @d) as d


Output:

dt                    dt2                     d
------------------    --------------------    --------
0x0000A13900000000    0x07000000000094360B    0x94360B


Now, I already understand from the documentation that datetime has a smaller range, and starts from 1753-01-01, while datetime2 and date use 0001-01-01 as their start date.

What I don't understand though, is that datetime appears to be little-endian while datetime2 and date are big-endian.  If that's the case, how can they even be properly sortable?

Consider if I want to know how many integer days are represented by a date type.  You would think you could do this:

declare @d date
set @d = '0001-01-31'
select cast(convert(varbinary, @d) as int)


But due to the endianness, you get 1966080 days!

To get the correct result of 30 days, you have to reverse it:

select cast(convert(varbinary,reverse(convert(varbinary, @d))) as int)


Or, of course you can do this:

select datediff(d,'0001-01-01', @d)


But that means internally somewhere it is reversing the bytes anyway.

So why did they switch endianness?

I only care because I'm working on a custom UDT in SQLCLR and the binary order of the bytes does seem to matter there, but these built-in types seem much more flexible.  Does SQL Server have something internal where each type gets to provide it's own sorting algorithm?  And if so, is there a way I can tap into that for my custom UDT? 

See also, a related (but different) question on StackOverflow.
SQL Server does not rely on the binary order for its "own" data types. For CLR datatypes you could use the iComparable interface, but as @MattJohnson mentioned, SQL Server ignores it:

http://connect.microsoft.com/SQLServer/feedback/details/252230/sqlclr-provide-the-ability-to-use-icomparable-or-a-similar-mechanism-for-udts



Microsoft does not publish the details about how the different datatypes are stored and worked with. However Books Online explicitly states that you cannot rely on a specific binary format for a specific datatype and that the format they use might change any time. So it is a good idea to store an INT as just that and not as VARBINARY, because you might not be able to read your data anymore after the next SP. 

As for the sorting: Most of the SQL Server core is written in C++. I assume internally a method similar to an iComparable is used. But again, there is no publicly accessible documentation about this available. Even if it were, you probably would not be able to exploit it because of the inherent differences between .NET and C++.