Change today's date, advancing one month and setting the systemtime - delphi

I would like a code sample for a function that takes a tDateTime and an integer as input and sets the system time using setlocaltime after advancing that tDateTime by (int) months. The time should stay the same.
pseudo code example
SetNewTime(NOW,2);
The issues I'm running into are rather frustrating. I cannot use incmonth or similar with a tDateTime, only a tDate, etc.

Below is a 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.

IncMonth should work with a TDateTime:
function IncMonth ( const StartDate : TDateTime {; NumberOfMonths : Integer = 1} ) : TDateTime;
Keep in mind a TDate is really just a TDateTime that by convention your ignore the fraction on.

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

setSystemTime uses UTC time, so you have to adjust for your time zone. The bias is the number of minutes your machine's timezone differs from UTC. This adjusts the date properly 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;

There isn't enough information to provide a definitive answer to your question.
Consider what you would want to happen if the day of the current month doesn't exist in your future month. Say, January 31 + 1 month. (7 months of the year have 31 days and the rest have fewer.) You have the same problem if you increment the year and the starting date is February 29 on a leap year. So there can't be a universal IncMonth or IncYear function that will work consistantly on all dates.
For anyone interested, I heartily recommend Julian Bucknall's article on the complexities that are inherent in this type of calculation
on how to calculate the number of months and days between two dates.
The following is the only generic date increment functions possible that do not introduce anomolies into generic date math. But it only accomplishes this by shifting the responsibility back onto the programmer who presumably has the exact requirements of the specific application he/she is programming.
IncDay - Add a or subtract a number of days.
IncWeek - Add or subtract a number of weeks.
But if you must use the built in functions then at least be sure that they do what you want them to do. Have a look at the DateUtils and SysUtils units. Having the source code to these functions is one of the coolest aspects of Delphi. Having said that, here is the complete list of built in functions:
IncDay - Add a or subtract a number of days.
IncWeek - Add or subtract a number of weeks.
IncMonth - Add a or subtract a number of months.
IncYear - Add a or subtract a number of years.
As for the second part of your question, how to set the system date & time using a TDatetime, the following shamelessly stolen code from another post will do the job once you have a 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;

Related

Work out number of days using Month Calendar Delphi

I'm using two Month calendar on my form. I select a date on each one and I want to calculate the number of days between these two days. I only code I have is
procedure TForm1.Button1Click(Sender: TObject);
Var
N, m: TDate;
d: Real;
l,k:String;
begin
N := (MonthCalendar1.Date);
m := MonthCalendar2.Date;
L := formatdatetime('dd', N);
K:=formatdatetime('dd',M);
d := StrToFloat(L)-StrToFloat(K);
ShowMessage(FloatToStr(d));
end;
You can use DaysBetween() from System.DateUtils to obtain the difference, in days, between two TDateTime values.
See Docs: http://docwiki.embarcadero.com/Libraries/Rio/en/System.DateUtils.DaysBetween
Edit, see code:
uses System.DateUtils;
[..]
procedure TForm1.Button1Click(Sender: TObject);
var
NumDays: Integer;
begin
NumDays := DaysBetween(MonthCalendar1.Date, MonthCalendar2.Date);
ShowMessage('Days between selected dates: ' + NumDays.ToString);
end;
You don't need a MonthCalendar to do this. The part of a TDateTime to the RHS of the decimal point is effectively a "day number" so if you substract one of those from another, you find the "days between". Apply a Trunc to both to discard the time-of-day parts. See http://docwiki.embarcadero.com/Libraries/Rio/en/System.TDateTime for definition of TDateTime.
So, for example, given
var
D1,
D2 : TDateTime;
DaysBetween : Integer;
and D1 and D2 are two TDateTimes (that you could enter if you wish using a TMonthCalendar), then
DaysBetween := Trunc(D2) - Trunc(D1);
The call to the built-in Trunc function discards the fractional, time-of-day part of a TDateTime value (that is, the partr to the right of the decimal point), so that 23:59 on one day and 00:01 on the next are calculated to be one day apart. This may, or may not, be the result you want, depending on your application, which is why I have suggested calculating it yourself rather than using the built-in DaysBetween function.

Is DateTimeToString in Delphi XE5 doesn't work?

I have small piece of code:
DateTimeToString(DiffString, 't.zzz', TDT);
ShowMessage('TDT: ' + DateTimeToStr(TDT));
ShowMessage('DiffString: ' + DiffString);
which result with first ShowMessage gives random nice DateTime TDT value...
second where DiffString is exacly 00:00.000
Could anyone check it in other IDE?
In fact DateTimeToString works just fine and is behaving exactly as designed. It is doing precisely what you asked it to.
Here is the SSCCE that you should have provided:
{$APPTYPE CONSOLE}
uses
SysUtils;
var
DiffString: string;
TDT: TDateTime;
begin
TDT := Date;
DateTimeToString(DiffString, 't.zzz', TDT);
Writeln('TDT: ' + DateTimeToStr(TDT));
Writeln('DiffString: ' + DiffString);
end.
Output:
TDT: 04/02/2014
DiffString: 00:00.000
The reason is, and I am guessing here, that your date time comes from a call to Date. Or perhaps your date time is an uninitialized variable.
Whichever way, it is clear that the time part is zero. Into DiffString you put the time and not the date. That is what the t.zzz format string means.
Try again with a date time containing a non-zero time:
{$APPTYPE CONSOLE}
uses
SysUtils;
var
DiffString: string;
TDT: TDateTime;
begin
TDT := Now;
DateTimeToString(DiffString, 't.zzz', TDT);
Writeln('TDT: ' + DateTimeToStr(TDT));
Writeln('DiffString: ' + DiffString);
end.
Output
TDT: 04/02/2014 11:16:43
DiffString: 11:16.942
Of course, t.zzz is a bad choice of format. It combines the short time format with milliseconds. As you can see, on my machine, the default short time format omits seconds. So you get hours, minutes and milliseconds. You'll need to re-think your format string. Perhaps 'hh:nn:ss.zzz' is what you need.

Add decimal value to a date time in firemonkey delphi XE4?

Is there anyone who can help me with, for example: Adding decimal 0,5 or 1 to a time?
eg. 0,5 + 15:30:50 = 16:00:50
If you have a time represented as a TDateTime, and wish to add a number of hours to it then you would do so like this:
uses
System.SysUtils, System.DateUtils;
function IncrementTimeBySpecifiedNumberOfHours(Time: TDateTime;
Hours: Double): TDateTime;
begin
Result := TimeOf(Time + Hours/HoursPerDay);
end;
I'm assuming that you are concerned only with times here and wish to implement a 24 hour wrap-around. Hence the use of TimeOf.

Add Time to a TDateTime

I would like to add seconds to a TDateTime variable, so that the result is the top of the minute. For example, if it's 08:30:25, I want change the TDateTime variable to store 08:31:00.
I see that TDateTime has a Decode function, which I could use. There isn't, however, an encode function to put the altered time back into a TDateTime variable.
Using DateUtils it's possible to do it like this:
Uses
DateUtils;
var
Seconds : Word;
Seconds := SecondOfTheMinute(MyTime); // Seconds from last whole minute
// Seconds := SecondOf(MyTime); is equivalent to SecondOfTheMinute()
if (Seconds > 0) then
MyTime := IncSecond(MyTime,60 - Seconds);
There sure is, at least in the recent versions - see the DateUtils unit, especially all the Recode* routines and EncodeDateTime. The DateUtils unit is already available in Delphi 2010, perhaps even in earlier version.
Theory
The TDateTime data type represents number of days since 30 Dec 1899 as a real number. That is, the integral part of TDateTime is an amount of whole days, and the fractional part represents a time of day.
Practical
Therefore, your problem could be solved using simple arithmetics:
var
Days: TDateTime;
Mins: Extended; { widen TDateTime's mantissa by 11 bits to accommodate division error }
begin
Days := Date + StrToTime('08:30:25');
Writeln(DateTimeToStr(Days));
Mins := Days * 24 * 60 ; // compute minutes
Mins := Math.Ceil(Mins); // round them up
Days := Mins / (24 * 60); // and back to days
{ or as simple and concise expression as: }
// Days := Ceil(Days * MinsPerDay) / MinsPerDay;
Writeln(DateTimeToStr(Days));

encodetime, incmilliseconds in delphi

i have create a coding to set a 20 millisecond faster than windows system. i'm using encodetime.
here's the code
procedure TForm1.Button1Click(Sender: TObject);
Var
delphi_datetime :tDateTime;
t_date : tdatetime ;
windows_datetime : tSystemTime;
begin
t_date := dATE;
delphi_datetime := encodetime(8,44,59,980);
delphi_datetime := incmillisecond(delphi_datetime, 20);
replacedate(t_date , delphi_datetime);
datetimetosystemtime( delphi_datetime , windows_datetime );
setlocaltime( windows_datetime );
showmessage('time now = ' + timetostr(delphi_datetime));
end;
aftr i run it, show the correct time. but the date goes to 30 dec 1899.. but i want to the current today date but with the time faster 20 milliseconds. any help.. please...
You have the arguments of ReplaceDate backward. It reads the date of the second parameter and assigns the date portion of the first parameter. The date portion of delphi_datetime is 0 because that's how EncodeTime works. You take that zero value and assign it to t_date, but then you continue working with delphi_datetime.
Reverse the arguments of ReplaceDate, and you should see that your current system time gets set to 8:45:00.000 with the current date.
ReplaceDate(delphi_datetime, t_date);
You could have noticed the mistake sooner if you hadn't used a separate t_date variable. If you'd called Date directly, your code would have failed to compile:
ReplaceDate(Date, delphi_datetime); // can't pass function result as "var" parameter
This works:
ReplaceDate(delphi_datetime, Date);
Rob has identified problems with your existing code. However your existing approach is needlessly complex. If you chose a simpler approach you would find it easier to get the code right.
If you want a date time that is 20 milliseconds greater than now, do it like this:
MyDateTime := IncMillisecond(Now, 20);
If you want a date time representing 0845 today, then you write:
MyDateTime := Date + EncodeTime(8, 45, 0, 0);

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