weekinmonth in powershell starts on Sunday…

This April month is full of surprises! Not only I did not have the opportunity to trick my colleagues on Sunday April first, but I had some bug of my code due to the start of the week.

[int](get-date).dayofweek and (Get-WmiObject Win32_LocalTime).dayofweek both refer Sunday as day 0.


PS> get-date
Wednesday, 11. April 2012 18:31:52
PS C:\svn> [int](get-date).dayofweek
3
PS C:\svn> (Get-WmiObject Win32_LocalTime).dayofweek
3

Consistently and unfortunately for me, the week does start on Sunday. And not on Monday.

To get week ending on Sunday, lets add 6 for Monday (5 for Tuesday, and so on) to dayofweek and get the 7-modulo

PS> (6+(get-date).dayofweek)%7
2

Ok, now Mon=0, Tue=1, Wed=2, etc…

Substract it from day of month to truncate to Monday

PS> (get-date).day - (6+(get-date).dayofweek)%7
9

First day of current week is Monday 9th

Now add 5 to get the first Day of month between 0 and 6
Divide by 7…
add 1 to get first week=1 (and not 0).

Truncate

PS> [math]::floor(((get-date).day - (6+(get-date).dayofweek)%7 + 5)/7)+1
3

Which slicely differs from weekinmonth when 1st of month is Sunday !

PS> (Get-WmiObject Win32_LocalTime).weekinmonth
2

Pfeww…

Published by Laurent Schneider

Oracle Certified Master

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2 Comments

  1. I wanted to run something on the first Friday of every month and used “weekinmonth” thinking that would be 1 for the first 7 days etc. Since it starts on Sunday, that was a mistake. For my purposes this works:

    $datetest = get-date
    $datetest = $datetest.AddDays(8)

    $datetest.Day
    $datetest.DayOfWeek

    ### instead of $datetest use the current date
    if($datetest.DayOfWeek -eq ‘Friday’ -and $datetest.Day -le 7) {
    “would run this is Friday”;
    }
    else { “not Friday the first week” }

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