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IsSameDay

Function IsSameDay(const AValue, ABasis: TDateTime): Boolean

Example​

procedure ScriptEvent(var Value: variant);
var
  ValueToCheck, BasisValue: TDateTime;
begin
  ValueToCheck := EncodeDateTime(2026, 7, 18, 23, 59, 0, 0);
  BasisValue := EncodeDateTime(2026, 7, 18, 8, 0, 0, 0);
  Value := IsSameDay(ValueToCheck, BasisValue); // True
end;

Usage​

IsSameDay reports whether one TDateTime falls in the numeric day interval selected by another value.

Parameters

NameTypeDescription
AValueTDateTime, constValue tested against the basis interval.
ABasisTDateTime, constValue whose truncated numeric day defines the interval.

Returns

True when AValue >= Trunc(ABasis) and AValue < Trunc(ABasis) + 1; otherwise False.

Behaviour

  • For ordinary dates on or after Velox's epoch, times from midnight inclusive to the following midnight exclusive match the basis day.
  • The arguments have different roles: ABasis selects the interval and AValue is tested, so this is not implemented as a symmetric component comparison.
  • No timezone conversion occurs.

Errors

There is no function exception handling. Invalid or non-finite encoded values can raise during decoding or produce non-useful comparisons rather than a meaningful Boolean.

Usage notes

Use the result as the documented field or date predicate only. It does not establish a timezone, business calendar or input provenance.

Additional Technical Info​

IsSameDay reports whether one TDateTime falls in the numeric day interval selected by another value.

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

Implementation

The DateUtils import binds directly to Delphi 37.0 System.DateUtils.IsSameDay. It truncates only ABasis, then applies two ordered comparisons to the raw TDateTime numbers. It does not decode and compare year/month/day components.

Edge cases and quirks

  • For timed dates before 30 December 1899, Delphi stores the time as the magnitude of a negative fraction. Raw ordered comparisons then do not describe the intended calendar-day interval; even comparing a pre-epoch non-midnight value with itself can return False. Decode and compare date components or use a sign-aware timestamp strategy for historical dates.
  • A NaN or otherwise non-finite value makes ordered comparisons non-useful rather than producing a meaningful date answer.
  • Do not swap arguments when code relies on the interval semantics, especially near the epoch.

Side effects

None.

Performance and concurrency

Constant-time field decoding or comparison with no external I/O. The routine uses no shared mutable state.

Related entries

  • IsToday uses this function with the host's current local Date.
  • DateOf removes the encoded time component with DateUtils semantics.

External references

Created 2026-07-15