performing Arithmetic on SYSTEMTIME

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)