UPDATE [ИтогНачислений] INNER JOIN 
WCdifference ON (ИтогНачислений.ДАТЫ_сдв >= WCdifference.dBegs)
 AND (ИтогНачислений.ДАТЫ_сдв < WCdifference.dEnds) 
SET [ИтогНачислений].ПоказМесWC = [WCdifference]![PokazDay]*[ИтогНачислений]![Days]
WHERE (((WCdifference.Id_dNo01)=True) 
AND (([WCdifference].[difference])> 0 ));

SELECT WaterCounDataC.Dregs, WaterCounDataC.DregsNext, 
DateAdd("m", 1 ,CDate("01." & Month([dregs]) & "." & Year([dregs]))) AS dBegs,
 CDate("01." & Month([dregsnext]) & "." & Year([dregsnext])) AS dEnds, 
CLng([dends]-[dbegs]) AS difference, IIf(Day([dregs])<> 1  Or 
Day([dregsnext])<> 1 ,True,False) AS Id_dNo01, CDate("01." & 
Month([dregs]) & "." & Year([dregs])) AS MonthDayBegs, 
[dBegs]-[dregs]- 1  AS daysBeg, CDate("01." & Month([dregsnext]) &
 "." & Year([dregsnext])) AS MonthDayEnds, Day([dregsNext]) AS daysEnd,
 WaterCounDataC.Raznica, WaterCounDataC.СнятУчет, 
[raznica]/CLng([DregsNext]-[Dregs]) AS PokazDay
FROM WaterCounDataC
WHERE (((WaterCounDataC.Dregs) Is Not Null) 
AND ((WaterCounDataC.DregsNext) Is Not Null));