Add business days to date in SQL without loops

Question

I currently have a function in my SQL database that adds a certain amount of business days to a date, e.g. if you enter a date that is a Thursday and add two days, it will r

Answer 1

I'm a little late to this party but I wound up writing my own version of this, because of drawbacks in the other solutions. Specifically this version addresses counting backwards, and starting on weekends.

There's an ambiguous situation that could arise, if you add zero business days to a weekend date. I've kept the date the same, but you can leave out this check if you always want to force a weekday to be returned.

CREATE FUNCTION [dbo].[fn_AddBusinessDays]
(
    @date datetime,
    @businessDays int
)
RETURNS datetime
AS
BEGIN
    --adjust for weeks first
    declare @weeksToAdd int = @businessDays / 7
    declare @daysToAdd int = @businessDays % 7

    --if subtracting days, subtract a week then offset
    if @businessDays < 0 begin
        set @daysToAdd = @businessDays + 5
        set @weeksToAdd = @weeksToAdd - 1
    end

    --saturday becomes zero using the modulo operator
    declare @originalDayOfWeek int = datepart(dw, @date) % 7
    declare @newDayOfWeek int = datepart(dw, dateadd(d, @daysToAdd, @date)) % 7

    --special case for when beginning date is weekend
    --adding zero on a weekend keeps the same date. you can remove the <> 0 check if you want Sunday + 0 => Monday
    declare @dateOffset int = case
        when @businessDays <> 0 and @originalDayOfWeek = 0 then 2
        when @businessDays <> 0 and @originalDayOfWeek = 1 then 1
        when @businessDays <> 0 and @newDayOfWeek < @originalDayOfWeek then 2
        else 0
    end

    -- Return the result of the function
    return dateadd(d, @daysToAdd + @dateOffset, dateadd(ww, @weeksToAdd, @date))

END