PowerShell Find Date of a previous day in any week

I often need to name a file with a date as part of the file name like:

ServerInventory12-18-2009.csv

A simple $date = $(get-date).ToString(“MM-dd-yyyy”) works great for today, but I recently had a need to create a file once a week, with a file name of the previous Saturday.

I had come across Arul Kumaravel’s WebLog post “How Can I Determine the Beginning and Ending Date of the Previous Month?”  (http://blogs.msdn.com/arulk/archive/2006/11/22/how-can-i-determine-the-beginning-and-ending-date-of-the-previous-month.aspx) when I was trying to find the first and last days of a month for a SQL query, and so I grabbed that bit of code and created this:

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([System.Datetime] $date = $(get-date)) $NumDaysSinceSaturday = $date.DayOfWeek.value__ + 1 $PreviousSaturday = `  $date.AddDays(– $NumDaysSinceSaturday).ToString(“MM-dd-yyyy”)

 

That worked great, but it’s good only for the previous Saturday. I could envision needing to get the previous Monday, or Friday, or even a day several weeks prior.

This function will do that:

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Function GetPreviousDate { param([STRING]$WeekDay = ` $(Throw ‘WeekDay is required’), [Int]$NumberOfWeeks = 0) $DayNumber = @{ “Saturday” = 1;  “Sunday” = 0;  “Monday” = -1; “Tuesday” = -2;  “Wednesday” = -3; “Thursday” = -4 “Friday” = -5} ([System.Datetime] $Today = $(get-date)) |Out-Null  $NumDaysSincePreviousDate = ` $Today.DayOfWeek.value__ + $DayNumber[$WeekDay] ([System.Datetime] $PreviousDateThisWeek ` = $Today.AddDays(– $NumDaysSincePreviousDate)) |Out-Null $PreviousDate =` $PreviousDateThisWeek.AddDays(–($NumberOfWeeks *7)).ToString(“MM-dd-yyyy”) }

 

Usage

. GetPreviousDate Friday 1

$PreviousDate Would show 12-11-2009 if I ran it the week of December 18th, 2009

. GetPreviousDate Friday

$PreviousDate Would show 12-18-2009 if I ran it the week of December 18th, 2009

. GetPreviousDate Wednesday 35

$PreviousDate Would show 04-15-2009 if I ran it the week of December 18th, 2009

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