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IncDay

Function IncDay(const AValue: TDateTime; const ANumberOfDays: Integer): TDateTime

Example​

procedure ScriptEvent(var Value: variant);
var
  InputValue: TDateTime;
begin
  InputValue := EncodeDateTime(2026, 7, 18, 9, 15, 30, 250);
  Value := IncDay(InputValue, 3); // 21 July 2026 at 09:15:30.250
end;

Usage​

IncDay adds a signed count of fixed 24-hour periods to a TDateTime through DateUtils timestamp arithmetic.

Parameters

NameTypeDescription
AValueTDateTime, constEncoded date/time to adjust. The original variable is not modified.
ANumberOfDaysInteger, constSigned number of whole days to add. Pass a negative value to subtract.

Returns

A newly encoded TDateTime after the requested adjustment.

Behaviour

  • The adjustment can cross minute, hour, date, month and year boundaries.
  • A zero count returns the same logical date/time, subject to the millisecond normalisation described below.
  • The operation treats TDateTime as an untagged numeric value. It does not discover or apply a timezone, daylight-saving transition or calendar-zone rule.
  • Velox exposes the exact two-argument declaration shown above. The count argument is required even if an upstream library documents a default or one-argument overload.

Errors

There is no function validation. Velox conversion, arithmetic or range exceptions propagate to the script if the input is not a valid finite encoded date/time or the result cannot be represented.

Usage notes

Use this routine when the required rule is an encoded fixed day offset. For a business rule expressed in local civil time, establish the timezone and daylight-saving policy outside this routine.

Additional Technical Info​

IncDay returns a new encoded date/time after adding a signed number of days. Positive counts move forward and negative counts move backward.

The example is fictional, deterministic and source-reviewed. It was not executed by the documentation workflow.

Implementation

The DateUtils import binds directly to Delphi 37.0 System.DateUtils.IncDay. That routine scales the requested day count to hours by 24 and ultimately delegates to IncMilliSecond, whose terminal converts through an integer millisecond timestamp.

Edge cases and quirks

  • This is fixed-duration arithmetic: each day represents exactly 24 hours. It is not a timezone-aware local-calendar adjustment.
  • Because the terminal converts through a millisecond timestamp, the incoming value is rounded to the nearest millisecond. Even a zero increment can therefore normalise sub-millisecond precision.
  • Intermediate scaling is performed before the timestamp terminal. An extreme ANumberOfDays can overflow during multiplication even when its declared type accepted the original argument.
  • Negative/pre-epoch TDateTime values use Delphi's sign-sensitive encoding. The timestamp conversion is designed to preserve chronological arithmetic, but direct numeric comparisons with the result need the normal pre-epoch precautions.
  • Floating-point TDateTime representation means the returned Double should not be tested for exact binary equality when a tolerance or component comparison is more appropriate.

Side effects

None.

Performance and concurrency

Constant-time arithmetic and conversion with no external I/O or shared mutable state.

Related entries

  • IncHour is the next routine in the implementation chain.
  • IncWeek provides a related date/time operation.

External references

Created 2026-07-15