WinAPI - Simple Operations with datetime

Introduction

When I started to use WinAPI datetime - I found many classes and several functions, but didn't find an explanation for simple operations with datetime. I describe here two operations:

1. Difference between two datetimes (datetime2-datetime1) and receiving weeks, days, hours, elapsed between them.
2. Add/Subtract weeks, days, hours, to/from datetime and receiving new datetime.

With WinAPI, we receive more than 29 thousand years for manipulation (start from 01.01.1601). To be more exact - 0x8000000000000000 100-nanoseconds intervals == from zero  [01 Jan 1601, 00:00:00] to 0x800...-1 [14 sep 30828, 02:48:05] where (1 sec = 1.000.000.000 nanoseconds).

WinAPI has two structs to store datetime - FILETIME and SYSTEMTIME. It was a surprise for me but SYSTEMTIME is only a representation of datetime and for real operations with datetime, we need to use FILETIME. (Why did Microsoft title it FILE? Where is the logic?)

SYSTEMTIME

Why do I say that SYSTEMTIME is only a representation? Look at this:

typedef struct  _SYSTEMTIME
    {
    WORD wYear;
    WORD wMonth;
    WORD wDayOfWeek;
    WORD wDay;
    WORD wHour;
    WORD wMinute;
    WORD wSecond;
    WORD wMilliseconds;
    } SYSTEMTIME;

What do you see? Right - nothing says to us that SYSTEMTIME is ONLY a representation. But if you receive datetime to SYSTEMTIME:

SYSTEMTIME st;
GetSystemTime (&st);

and make such operation as add 100 days:

st.wDay += 100;

and then try to receive string representation of this new date:

GetDateFormat
 (
  LOCALE_USER_DEFAULT,
  NULL,
  &st,
  "yyyy MM dd",
  buff,     /*char buff[20];*/
  20
 );

You receive nothing. GetLastError returns number of error, GetDateFormat returns 0. Of course, if you assign to wDay number valid for wMonth, you receive expected result (example - for Feb 2003 valid days from 1 to 28, if you assign wDay=29 you receive nothing). So, conclusion - SYSTEMTIME is not for the above mentioned operations.

Ok, let us move to FILETIME. FILETIME contains two DWORD variables:

typedef struct  _FILETIME
    {
    DWORD dwLowDateTime;
    DWORD dwHighDateTime;
    } FILETIME;

Two DWORDs allow FILETIME to store 64 bit values. Value of FILETIME is equal to number of 100-nanosecond intervals since 01 Jan 1601, 00:00:00. So, if (FILETIME == 0) than we have 01 Jan 1601, 00:00:00. If (FILETIME == 10000000*60*60*24) we have 02 Jan 1601, 00:00:00.
(1 sec = 10000000 (100 nanoseconds) - in one min 60 secs, in one hour 60 mins and in one day 24 hours :)

64 bit value means that we can store 0xFFFFFFFFFFFFFFFF intervals! But manipulate only 0x800..., in two times less. This restriction is superimposed by FileTimeToSystemTime. Read help:

The FileTimeToSystemTime function only works with FILETIME values that are less than 0x8000000000000000.

I think internals of this function use double (for convert to store double we need SIGNED __int64), and we lose 1 bit to sign, and receive in two times less max value. But I think it is not a tragedy :) - 29 thousand is enough.

Go to Algorithms

/*declare const that represent week,day,hour,minute,second*/

const __int64 nano100SecInWeek=(__int64)10000000*60*60*24*7;
const __int64 nano100SecInDay=(__int64)10000000*60*60*24;
const __int64 nano100SecInHour=(__int64)10000000*60*60;
const __int64 nano100SecInMin=(__int64)10000000*60;
const __int64 nano100SecInSec=(__int64)10000000; 
  
/*1. Difference BETWEEN two datetime 
////////////
//equivalent of DATEDIFF function from SQLServer
//Returns the number of date and time boundaries crossed 
//between two specified dates. 
////////////*/
double     /* return count of period between pst1 and pst2*/
DT_PeriodsBetween
 (
 const __int64 datepart, /*datepart that we want to count, 
      {nano100SecInDay ...}*/
 const SYSTEMTIME* pst1, /*valid datetime*/
 const SYSTEMTIME* pst2  /*valid datetime*/
 )
{
 FILETIME ft1,ft2;
 __int64 *pi1,*pi2;
 
 /*convert SYSTEMTIME to FILETIME
 //SYSTEMTIME is only representation, so need convert to 
 //FILETIME for make some calculation*/
 SystemTimeToFileTime (pst1,&ft1);
 SystemTimeToFileTime (pst2,&ft2); 
 /*convert FILETIME to __int64
 //FILETIME is struct with two DWORD, for receive 
 //ability calculation we must reference to FILETIME as to int*/
 pi1 = (__int64*)&ft1;
 pi2 = (__int64*)&ft2; 
 /*compare two datetimes and (bigger date - smaller date)/period*/
 return (CompareFileTime(&ft1,&ft2)==1) ? 
    (((*pi1)-(*pi2))/(double)datepart) : (((*pi2)-(*pi1))/(double)datepart);
} 
  
/*2. Add/Subtract weeks,days,hours... to/from datetime 
    //and receiving new datetime. 
////////////
//equivalent of DATEADD function from SQLServer
//Returns a new datetime value based on adding an interval
// to the specified date.
////////////*/
SYSTEMTIME /*new datetime*/
DT_AddDiff
 (
 const __int64 datepart, /*datepart with we want to manipulate, 
    {nano100SecInDay ...}*/
 const __int64 num, /*value used to increment/decrement datepart*/
 const SYSTEMTIME* pst /*valid datetime which we want change*/
 )
{
 FILETIME ft;
 SYSTEMTIME st;
 __int64* pi; 

 SystemTimeToFileTime (pst,&ft); 
 pi = (__int64*)&ft; 
 (*pi) += (__int64)num*datepart; 

 /*convert FAILETIME to SYSTEMTIME*/
 FileTimeToSystemTime (&ft,&st); 

 /*now, st contain new valid datetime, so return it*/
 return st;
} 

SAMPLE Use of Functions

/*1*/
SYSTEMTIME s1={2003,11,0,28,12/*hours*/,0,0,0},s2={2003,12,0,1,0,0,0,0};
printf("%f\n",DT_PeriodsBetween(nano100SecInDay,&s1,&s2));

/*2*/
char buff[100];
s1=DT_AddDiff(nano100SecInDay,-1,&s2); //diff 1 day from s2

/*WinAPI function GetDateFormat uses for receive string with
  datetime in specified format*/
GetDateFormat
(
 LOCALE_USER_DEFAULT,
 NULL,
 &s1,
 "yyyy MM dd", //my format
 buff,
 100
);
printf("20031201 - 1 day = %s\n",buff);

These functions can't be used for work with month, quarter, year because it they are not constants. For work with such time intervals, you need to modify the above algorithms.

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