In Delphi: How do I round a TDateTime to closest second, minute, five-minute etc?

Something like that (completely untested, written directly in browser):

function RoundToNearest(time, interval: TDateTime): TDateTime;
var
  time_sec, int_sec, rounded_sec: int64;
begin
  time_sec := Round(time * SecsPerDay);
  int_sec := Round(interval * SecsPerDay);
  rounded_sec := (time_sec div int_sec) * int_sec;
  if (rounded_sec + int_sec - time_sec) - (time_sec - rounded_sec) then
    rounded_sec := rounded_sec + time_sec;
  Result := rounded_sec / SecsPerDay;
end;

The code assumes you want rounding with second precision. Milliseconds are thrown away.

Wow! guys, how do you complicate too much something so simple... also most of you loose the option to round to nearest 1/100 second, etc...

This one is much more simple and can also round to milisenconds parts:

function RoundToNearest(TheDateTime,TheRoundStep:TDateTime):TdateTime;
    begin
         if 0=TheRoundStep
         then begin // If round step is zero there is no round at all
                   RoundToNearest:=TheDateTime;
              end
         else begin // Just round to nearest multiple of TheRoundStep
                   RoundToNearest:=Round(TheDateTime/TheRoundStep)*TheRoundStep;
              end;
    end;

You can just test it with this common or not so common examples:

// Note: Scroll to bottom to see examples of round to 1/10 of a second, etc

// Round to nearest multiple of one hour and a half (round to 90'=1h30')
ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(1,30,0,0))
                          )
           );

// Round to nearest multiple of one hour and a quarter (round to 75'=1h15')
ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(1,15,0,0))
                          )
           );

// Round to nearest multiple of 60 minutes (round to hours)
ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(1,0,0,0))
                          )
           );

// Round to nearest multiple of 60 seconds (round to minutes)
ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(0,1,0,0))
                          )
           );

// Round to nearest multiple of second (round to seconds)
ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(0,0,1,0))
                          )
           );

// Round to nearest multiple of 1/100 seconds
ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,141)
                                         ,EncodeTime(0,0,0,100))
                          )
           );

// Round to nearest multiple of 1/100 seconds
    ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(0,0,0,100))
                          )
           );

// Round to nearest multiple of 1/10 seconds
    ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,151)
                                         ,EncodeTime(0,0,0,10))
                          )
           );

// Round to nearest multiple of 1/10 seconds
    ShowMessage(FormatDateTime('hh:nn:ss.zzz'
                          ,RoundToNearest(EncodeTime(15,31,37,156)
                                         ,EncodeTime(0,0,0,10))
                          )
           );

Hope this helps people like me, that need to round to 1/100, 1/25 or 1/10 seconds.