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WithinPastMonths

Function WithinPastMonths( const ANow, AThen : TDateTime; const AMonths : Integer) : Boolean

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := WithinPastMonths(
    EncodeDate(2024, 1, 1), EncodeDate(2024, 12, 31), 11);
  // Value is True: 365 days contains 11 complete 30.4375-day periods.
end;

Usage​

WithinPastMonths reports whether two date/time values are within a supplied count of complete approximate 30.4375-day months.

Parameters

NameTypeDescription
ANowTDateTimeOne encoded date/time endpoint. It need not be the later or current value.
AThenTDateTimeThe other encoded endpoint.
AMonthsIntegerInclusive maximum count of complete 30.4375-day periods. Negative values produce False.

Returns

True when MonthsBetween(ANow, AThen) <= AMonths; otherwise False.

Behaviour

The result is symmetric and duration-based. A 30-day interval contains 0 complete approximate months; 30 days 10 hours 30 minutes is exactly one such month. Moving from one calendar date to the same day in the next month does not guarantee a result of 1 because actual month lengths vary.

Errors

Velox does not catch RTL conversion or arithmetic failures from invalid/non-representable inputs.

Additional Technical Info​

Reports whether two encoded date/time values are no more than AMonths complete approximate months apart. Here a month is a fixed average duration of 30.4375 days; calendar month boundaries and matching day numbers are not used.

Implementation

Velox binds directly to System.DateUtils.WithinPastMonths. MonthsBetween converts each endpoint to a millisecond timestamp, takes the absolute distance and divides it by Round(86,400,000 * 30.4375), or 2,629,800,000 milliseconds. Integer division discards the remainder before the inclusive comparison.

Edge cases and quirks

  • This is not calendar-month or anniversary logic. Use IncMonth plus an explicit comparison when calendar boundaries are the requirement.
  • The floor operation is deliberately permissive: 2.9 approximate months passes a limit of 2.
  • Negative limits return False, even for equal values.
  • Inputs are normalised to milliseconds and no timezone/daylight-saving conversion occurs.
  • Free Pascal exposes the same family and average-duration model; installed Delphi 37.0 constants and arithmetic define Velox behaviour.

Performance and concurrency

The calculation is constant-time integer arithmetic with no allocation, host-clock read or shared Velox state.

Related entries

  • MonthsBetween — returns the complete approximate-month count used here.
  • WithinPastYears — uses the corresponding 365.25-day year approximation.
  • IncMonth — performs calendar-aware month movement and has different end-of-month semantics.

External references

Created 2026-07-15