performing Arithmetic on SYSTEMTIME - windows

Question

I have a time value represented in SYSTEMTIME, i want to add/subtract 1 hour from it and get the newly obtained SYSTEMTIME. I want the conversion should take care of the date change on addition/subtraction or month change or e1 year change .
Can someone help me with this if there is some windows api which does arithmetic on SYSTEMTIME

Answer 1

If you're using C# (or VB.NET, or ASP.NET) you can use
DateTime dt = DateTime.Now.AddHours(1);
You can use negative numbers to subtract:
DateTime dt = DateTime.Now.AddHours(-1);
EDITED:
I extract an asnwer from this post
They suggest converting SYSTEMTIME to FILETIME, which is a number of
ticks since an epoch. You can then add the required number of 'ticks'
(i.e. 100ns intervals) to indicate your time, and convert back to
SYSTEMTIME.
The ULARGE_INTEGER struct is a union with a QuadPart member, which is
a 64bit number, that can be directly added to (on recent hardware).
SYSTEMTIME add( SYSTEMTIME s, double seconds ) {
FILETIME f;
SystemTimeToFileTime( &s, &f );
ULARGE_INTEGER u ;
memcpy( &u , &f , sizeof( u ) );
const double c_dSecondsPer100nsInterval = 100. * 1.E-9;
u.QuadPart += seconds / c_dSecondsPer100nsInterval;
memcpy( &f, &u, sizeof( f ) );
FileTimeToSystemTime( &f, &s );
return s;
}
If you want to add an hour use SYSTEMTIME s2 = add(s1, 60*60)

Answer 2

To add signed seconds (forward or backward in time) in C++:
const double clfSecondsPer100ns = 100. * 1.E-9;
void iAddSecondsToSystemTime(SYSTEMTIME* timeIn, SYSTEMTIME* timeOut, double tfSeconds)
{
union {
ULARGE_INTEGER li;
FILETIME ft;
};
// Convert timeIn to filetime
SystemTimeToFileTime(timeIn, &ft);
// Add in the seconds
li.QuadPart += tfSeconds / clfSecondsPer100ns;
// Convert back to systemtime
FileTimeToSystemTime(&ft, timeOut);
}

Answer 3

#include <stdio.h>
#include <windows.h>
#define NSEC 60*60
main()
{
SYSTEMTIME st;
FILETIME ft;
// Get local time from system
GetLocalTime(&st);
printf("%02d/%02d/%04d %02d:%02d:%02d\n",
st.wDay,st.wMonth,st.wYear,st.wHour,st.wMinute,st.wSecond);
// Convert to filetime
SystemTimeToFileTime(&st,&ft);
// Add NSEC seconds
((ULARGE_INTEGER *)&ft)->QuadPart +=(NSEC*10000000LLU);
// Convert back to systemtime
FileTimeToSystemTime(&ft,&st);
printf("%02d/%02d/%04d %02d:%02d:%02d\n",
st.wDay,st.wMonth,st.wYear,st.wHour,st.wMinute,st.wSecond);
}