On the various ways of getting the current time and date in Win32

Raymond Chen

There are a number of functions in Win32that obtain the current date and time.Here’s how they fit together:

The starting point isGet­System­Time­As­File­Time.This returns the current time in UTC in the form of aFILE­TIME structure.This also happens to be the time format used internally by the system,so this value can be retrieved with a minimum of fuss.

You can also callGet­System­Time which returns the current UTC timein the form of aSYSTEM­TIME structure.To do this, the operating system takes thecurrent FILE­TIME and then callsthe moral equivalent ofFile­Time­To­System­Time,whichdoes a boatload of gnarly mathto decompose theFILE­TIMEinto year, month, day, hour, minute, second, and millisecond.

Meanwhile, you can also get the current local time by takingtheFILE­TIME returned byGet­System­Time­As­File­Time,then passing it toFile­Time­To­Local­File­Time.

And finally, there’sGet­Local­Time,which does the same thingasGet­System­Time,but it starts with the local file time.

In equations:

FormatTime zoneFunctionAlgorithm
FILE­TIMEUTCGet­System­Time­As­File­Time(Native format)
FILE­TIMELocal(None)Get­System­Time­As­File­Time + File­Time­To­Local­File­Time
SYSTEM­TIMEUTCGet­System­Time Get­System­Time­As­File­Time+ File­Time­To­System­Time
SYSTEM­TIMELocalGet­Local­Time Get­System­Time­As­File­Time + File­Time­To­Local­File­Time+ File­Time­To­System­Time

I happen to be a fan of commutative diagrams.(Though since there are no closed loops, there is nothing to commute.)

A 2-by-2 grid of boxes. The top row is labeled FILE­TIME; the bottom row is labeled SYSTEM­TIME. The first column is labeled UTC; the second column is labeled Local. The upper left box is labeled Get­System­Time­As­File­Time. There is an outgoing arrow to the right labeled File­Time­To­Local­File­Time leading to the box in the second column labeled None. There is an outgoing arrow downward labeled File­Time­To­System­Time leading to the box in the second row, first column, labeled Get­System­Time. From the box in the upper right corner labeled None, there is an outgoing arrow downward labeled File­Time­To­System­Time leading to the box in the second row, second column, labeled Get­Local­Time.

To complete the commutative diagram,there would be an arrow connecting the bottom two boxes calledSystem­Time­To­Local­Time,but there is no such function.

Today’s article was inspired by some code I ran across which did this:

SystemTimeToFileTime(&stNow, &ftNow);

That code unwittingly takes an excursion fromGet­System­Time­As­File­TimethroughFile­Time­To­System­Time toGet­System­Time,then back throughSystem­Time­To­File­Time&shy toreturn toGet­System­Time­As­File­Time,just so that it can end up where it started,but with a lot of extra math (and loss of resolution).

Exercise: How would you implement theSystem­Time­To­Local­Timefunction?