fprintf(fp, "time_saved %lu\n", (UINT) time( NULL ) );
   // convert it to unsigned long so that you can use it in sprintf

Then output the string to the file.

When you want to read it in again you could input the line and then use

{
 time_t time_saved=0;
 UINT u=0;
 fscanf( fp, "%*s%lu", &u);
  // read it in as an unsigned long as sscanf can't read to __int64 variables directly
 time_saved=u;
}

(or alternatively use sscanf and sprintf to write to / read from a string, advance along the string as one does so, and then save the string to a file).

At least that is how one might do it if one wanted to use a text format save rather than a binary one. In a binary save you would probably not need to do any conversion but could just write out the variable in binary using sizeof() to find its size.

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See it in action

The zip for download includes a MSVC project to make a tiny app that just displays the current time - see the screen shot at the head of this page. You can change the date on your calender and see how it responds - but first a few warnings are in order.

Note some programs with 30 day trial periods can behave crankily if you change the system time - may tell you that the trial period is over even after you adjust the time back again - so make sure you have no such programs running when you do it).

Others may crash if you go beyond 2038. That's because time_t has to be non negative for the windows implementation of most of the time routines. If you call them with negative values for time_t then you may get an access violation. See Problems Running Windows Media Player 7 or 7.1 with a Date that Is Later than January 18, 2038

Others may behave strangely if you change the date. For instance, probably it is best to close your browser in case its history folder gets confused.

So, close anything that might be sensitive to the date before varying the calender date to test this library. If in doubt, use Ctrl Alt + Delete and close everything there except Explorer.

Check out my Activity Timer program for an example of an app that uses this library

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Source code

So anyway without further ado, here it is.

time64.zip

Includes the time64.c and time64.h files. Also includes a MSVC project with an example windows program using it.

The example program is a c++ one made simply by modifying the Hello World app to show the time. It has a c++ version of the source code: time64.cpp which differs only in the headers included at the head of the file.

This is free source - do with it as you like. Treat it as you would a demo program in a c programming book.

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Dates before 1980

This library supports negative 64 bit time_t. However, you can't test it by setting the system time. Instead, try replacing the WM_PAINT call with

hdc = BeginPaint(hWnd, &ps);
RECT rt;
t=-10000000000;
tm *today=localtime(&t);...

and you will find that it shows a time in the 17th century.

This could make the library useful for anyone who wants to work with times from 17th century onwards and display them, and find the differences between two such times etc.

Another place where it may be useful - you can use negative time_t values in Unix to represent dates in the twentieth century, but you can only use positive time_t in Windows, so this code could be helpful if you want to port code from Unix.

You can't set the Windows system time to any date earlier than 1980 - on Win 98 anyway. That's the earliest you can set it using the calender, and the same limitation applies if you try to do it calling the routines in the API.

Here is how one would do it to try to set the system time to the early 17th century:
(usual cautions about setting the system time - and additional caution about possible crashes, as with setting it to later than 2038). You have to enable system-time priviledge to do it.

 

 time_t  t=-10000000000;
 SYSTEMTIME st;
 T64ToSystemTime(&t,&st);
 /* enable system-time privilege, set time, disable privilege */
 HANDLE hToken;
 TOKEN_PRIVILEGES tp; 
 LUID             luid;      
 OpenProcessToken
  ( GetCurrentProcess(),TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken ) ;
 LookupPrivilegeValue( NULL, SE_SYSTEMTIME_NAME /***"SeSystemTimePrivilege"**/, &luid ); 
 tp.PrivilegeCount           = 1; 
 tp.Privileges[0].Luid       = luid; 
 tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED; 
 if(ERROR_SUCCESS==AdjustTokenPrivileges( hToken, FALSE, &tp, sizeof(TOKEN_PRIVILEGES), NULL, NULL ))
 {
  if(!SetLocalTime(&st))
  {
   ErrorHandler("SetLocalTime");
  }
  AdjustTokenPrivileges( hToken, TRUE, &tp,sizeof(TOKEN_PRIVILEGES), NULL, NULL );                  
 }

If this works, then when you use this library (though not normally of course)

t=time(NULL);

should give a time in the early 17th century.

But it doesn't work, at least not in Windows 98. You get an error message here that the parameter for SetLocalTime() is incorrect in Windows 98 if you try to set the date to any system date before 1980.

The error handler here is the standard:

#include <stdio.h>
void ErrorHandler(char *szError)
{
#define ONE_K 1024
char szMsg[ONE_K];
LPVOID lpMsgBuf;
static char in_this;
if(in_this)
return;
in_this=1;
FormatMessage
(FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS
,NULL
,GetLastError()
,MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT)
,(LPTSTR) &lpMsgBuf
,0
,NULL
);
sprintf(szMsg,"%s\n\n%s",szError,lpMsgBuf);
MessageBox(NULL,szMsg,"Error",MB_ICONEXCLAMATION|MB_TOPMOST);
LocalFree( lpMsgBuf );
in_this=0;
}

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Changes

Contact e-mail:

Robert Walker support@tunesmithy.co.uk

August 14th

Original version of T64ToFileTime modified its t64 argument. New version:

static void T64ToFileTime(t64 *pt,FILETIME *pft)
{
 LARGE_INTEGER li;  
 li.QuadPart=*pt*SECS_TO_FT_MULT;
 pft->dwLowDateTime=li.LowPart;
 pft->dwHighDateTime=li.HighPart;
}

July 30th

First Release.

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