We consider two approaches to adjustment for length of month variation in nonnegative flow time series observed monthly. One approach is to divide the observed series value in each month by the length of that month and then multiply all series values by the average length of month (30.4375). The other approach is to include length of month as an explanatory variable in a regression model with ARIMA time series errors (REGARIMA model), and then estimate and remove the length of month effect.