How to determine whether the OS has the latest DST rules

[Dave Cantor]

On 06-Nov-2008, Nicolas Alvarez wrote:

> yoshito_umaoka at us.ibm.com wrote:

> > To access time zone rules used by Windows, you can use the API
> > below -

> http://msdn.microsoft.com/en-us/library/system.timezoneinfo.getadj
> ustmentrules.aspx

> Any way without .NET?

Yup.  Here's a start:

dosprompt> REG QUERY 
"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows 
NT\CurrentVersion\Time Zones\Eastern Standard Time"

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows 
NT\CurrentVersion\Time Zones\Eastern Standard Time    
Display     REG_SZ  (GMT-05:00) Eastern Time (US & Canada)    
Dlt REG_SZ  Eastern Daylight Time    
Std REG_SZ  Eastern Standard Time    
MapID       REG_SZ  38,39    
Index       REG_DWORD       0x23    
TZI REG_BINARY      
2C01000000000000C4FFFFFF00000B000000010002000000000000000000030000
0002000200000000000000

You can decode the item TZI.

2C01000000000000C4FFFFFF00000B000000010002000000000000000000030000
0002000200000000000000

First there are three 32-bit numbers:

as shown       reverse bytes    translate
2C010000  -->  0000012c --> 300  minutes offset from UTC
00000000       00000000 --> 0 std time offset
C4FFFFFF       ffffffc4 --> -60 dst offset

That is timezone offset is 300 minutes;
offset for standard time is 300 minutes;
offset for  daylight time is (300 - 60) = 240 minutes

Then there are two 28-byte SYSTEMTIME structures

00000B00000001000200000000000000
00000300000002000200000000000000

The first is the change date to standard time
The second is the change date to daylight time

0000 0B00 0000 0100 0200 0000 0000 0000  < ch to std tim
0000 0300 0000 0200 0200 0000 0000 0000  < ch to dst
yr   mon  dow  day  hr   min  sec  msec

For each item reverse the bytes.

  0   0b   0    01    02  0     0    0
  0   03   0    02    02  0     0    0

Year is always 0

This says change to standard time on the first Sunday in November 
at 02:00:00.000 and change to daylight time on the second Sunday 
in March at 02:00:00.000 .


I hope this helps you.

Dave C.