WinAPI - Simple Operations with datetime






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Dec 1, 2003
3 min read

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Simple operations with datetime - add/diff to/from datetime, difference between datetime on WinAPI
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:
- Difference between two datetimes (
datetime2-datetime1
) and receiving weeks, days, hours, elapsed between them. - 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 struct
s 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 DWORD
s 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 withFILETIME
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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