Change today by advancing one month and setting system time

Question

I need some sample code for a function that takes tDateTime and an integer as input and sets the system time using setlocaltime after moving that tDateTime to (int) months. Time must remain unchanged.

pseudo code example

SetNewTime(NOW,2); 

The problems I run into are pretty disappointing. I cannot use incmonth or the like with tDateTime, only tDate, etc.

Answer 1

Below is the complete command line program that works for me. Tested in Delphi 5 and 2007. Why do you say IncMonth does not work for TDateTime?

 program OneMonth; {$APPTYPE CONSOLE} uses SysUtils, Windows, Messages; procedure SetLocalSystemTime(settotime: TDateTime); var SystemTime : TSystemTime; begin DateTimeToSystemTime(settotime,SystemTime); SetLocalTime(SystemTime); //tell windows that the time changed PostMessage(HWND_BROADCAST,WM_TIMECHANGE,0,0); end; begin try SetLocalSystemTime(IncMonth(Now,1)); except on E:Exception do Writeln(E.Classname, ': ', E.Message); end; end. 

Answer 2

Based on your pseudo code:

 procedure SetNewTime(aDateTime: TDateTime; aMonths: Integer); var lSystemTime: TSystemTime; begin DateTimeToSystemTime(aDateTime, lSystemTime); Inc(lSystemTime.wMonth, aMonths); setSystemTime(lSystemTime); end; 

Answer 3

IncMonth should work with TDateTime :

 function IncMonth ( const StartDate : TDateTime {; NumberOfMonths : Integer = 1} ) : TDateTime; 

Keep in mind that TDate is actually just TDateTime, which, by convention, ignores the fraction.

Answer 4

setSystemTime uses UTC time, so you need to set your time zone. Bias is the number of minutes that your time zone differs from UTC. This correctly sets the date on my system:

 procedure SetNewTime(aDateTime: TDateTime; aMonths: Integer); var lSystemTime: TSystemTime; lTimeZone: TTimeZoneInformation; begin GetTimeZoneInformation(lTimeZone); aDateTime := aDateTime + (lTimeZone.Bias / 1440); DateTimeToSystemTime(aDateTime, lSystemTime); Inc(lSystemTime.wMonth, aMonths); setSystemTime(lSystemTime); end; 

Answer 5

There is not enough information to give a definitive answer to your question.

Think about what you would like if the day of the current month did not exist in your next month. Say January 31 + 1 month. (7 months of the year 31 days, while the rest have less). You have the same problem if you increase the year, and the starting date is February 29 in a leap year. Thus, there cannot be a universal IncMonth or IncYear function that will work continuously on all dates.

For anyone interested, I heartily recommend Julian Bucknall 's article on the complexities inherent in this type of computation on how to calculate the number of months and days between two dates.

Listed below are the only common date increment functions that do not introduce anomalies in general date math. But it only does this, shifting the responsibility back to the programmer, who supposedly has exact requirements for the specific application that he / she is programming.

IncDay - Add or subtract a few days.
IncWeek - Add or subtract a few weeks.

But if you have to use the built-in functions, then at least make sure that they do what you want them to do. Take a look at the DateUtils and SysUtils modules. Having source code for these features is one of the coolest aspects of Delphi. Having said that, here is a complete list of built-in functions:

IncDay - Add or subtract a few days.
IncWeek - Add or subtract a few weeks.
IncMonth - Add or subtract a few months.
IncYear - add or subtract a few years.

Regarding the second part of your question, how to set the system date and time using TDatetime, the following shamelessly stolen code from another message will do the job after you have TDatetime that has the value you want:

 procedure SetSystemDateTime(aDateTime: TDateTime); var lSystemTime: TSystemTime; lTimeZone: TTimeZoneInformation; begin GetTimeZoneInformation(lTimeZone); aDateTime := aDateTime + (lTimeZone.Bias / 1440); DateTimeToSystemTime(aDateTime, lSystemTime); setSystemTime(lSystemTime); end;