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Value

property Value: TDateTime read write

Example​

procedure ScriptEvent(var Value: variant);
var
  DateTimeValue: TDateTimeField;
begin
  DateTimeValue := TDateTimeField(Dataview.Query.FieldByName('ProcessedAt'));
  if DateTimeValue.IsNull then
    Value := Null
  else
    Value := DateTimeValue.Value;
end;

Usage​

Gets or sets the current record's typed TDateTime value, with a null read collapsing to numeric zero.

Behaviour

  • Reading follows the dataset cursor; navigation changes the represented record.
  • Writing zero stores the real zero date/time. It does not clear/null the field.
  • Use Clear to write null.
  • For TDateField and TTimeField, the same property is but the provider/Velox field represents only date or time and can discard the other component.
  • Posting the dataset makes the edit part of its in-memory change set; external persistence still depends on ApplyUpdates/save/transaction/flow success.

Errors

Writes can raise when the dataset is missing, inactive/read-only/not editing, the field is read-only/calculated, validation or constraints fail, conversion is unsupported, or the provider/database rejects the value. Reads can raise for invalid/destroyed dataset state.

Additional Technical Info​

Value reads or writes the current record's date/time as Delphi TDateTime. It is the typed version of AsDateTime. A null read returns numeric zero, so always check IsNull or use AsVariant when null must remain distinct from 30 December 1899.

The example preserves that distinction and retains no field reference beyond the event. The cast requires a configured TDateTimeField/date/time descendant. It is source-reviewed and was not executed by the documentation workflow.

TDateTime is an eight-byte Double: whole-number days are relative to 30 December 1899 and the fraction represents time of day. It contains no timezone or offset.

Implementation

When the reusable I/O buffer exists and the field is not currently validating, the getter first writes Double zero to that buffer. It then calls the owning dataset's GetFieldData in converted/non-native mode. When data exists, it reads a Double from the buffer. When the dataset reports null, GetAsDateTime returns zero. AsFloat delegates to the same getter; AsVariant instead returns a date Variant or Null.

The setter writes the supplied Double to that I/O buffer and calls SetFieldData in converted/non-native mode. The field class performs no local range, finite-value, timezone or subtype check before delegating. The dataset/provider converts to its native representation and applies edit-state, validation, constraints and change events.

Edge cases and quirks

  • Null and 30 December 1899 compare equal through Value alone. A default 0 variable can therefore hide missing data.
  • TDateTime is a civil numeric value, not an instant. Arithmetic does not account for timezones, daylight-saving gaps/overlaps or calendar business rules.
  • NaN, infinity and out-of-range values are not rejected by this setter itself; failures or corrupt-looking output can occur later in formatting/provider/database code.
  • Floating-point fractions can carry sub-millisecond approximation. Database/provider precision may round or truncate further.
  • A retained field reference becomes invalid when the dataset/schema is destroyed or rebuilt; it also continues to follow the mutable current row.

Side effects

Reading calls dataset field access only. Writing mutates the current record buffer and can trigger field validation, calculated/dependent-field work and dataset change events. It does not itself post, apply or commit.

Performance and concurrency

Typed access is constant-time. The field, I/O buffer and cursor are unsynchronised and must remain in one owning Velox flow/thread.

Remarks

Preserve IsNull, define the integration's timezone convention separately and let the surrounding Velox lifecycle own edit/post/apply/transaction decisions.

External references

Created 2026-07-15