Use PowerShell to determine the first day of the current calendar quarter.

Time for some fun with PowerShell.

A colleague of mine has PowerShell script that queries a bunch of data from the last 90 days. The intention of the script is to only collect data from the current quarter, but depending on when the script is run the last 90 days could include time (and subsequently data) from the previous quarter.

I was about to recommend he use the first day of the current calendar quarter as the start date for his query versus the day 90 days ago, but realized this would require some code so I set out to find some. Not finding anything relevant with my BingFu, I came up with this initial code:

[code language="powershell"]
$Today = Get-Date
$Quarter = [Math]::Ceiling($Today.Month / 3)
Switch ($Quarter) {
1 {
$StartDate = "1/1/" + $Today.Year.ToString()
}
2 {
$StartDate = "4/1/" + $Today.Year.ToString()
}
3 {
$StartDate = "7/1/" + $Today.Year.ToString()
}
4 {
$StartDate = "10/1/" + $Today.Year.ToString()
}
}
[/code]

This code does the following:

1. Grab the current date.
2. Take the current month, divide it by 3 (dividing 12 months by 3 gives us 4 quarters of 3 months), and round-up any fractional number by using the Ceiling method in the Math operator (so 0.6 becomes 1, 1.33 becomes 2, etc...).
3. Use a Switch construct to take the rounded up number stored in the $Quarter variable and select the right month and day, always appending the current Year to the pre-determined Month/Day combination.

Checking $StartDate shows that no matter what was used for $Today's date, the correct first day of the corresponding calendar quarter was used. However... That Switch construct seemed like a little bit overkill (and I was hard coding the month) for something that should be simple, so I thought long and hard about how to streamline it further and came up with this replacement one-liner for the Switch construct:

[code language="powershell"]
$StartDate = "$(($Quarter * 3 - 2))/1/" + $Today.Year.ToString()
[/code]

This code multiplies the rounded up Quarter number by 3 (so 1 becomes 3, 2 becomes 6, etc..), then subtracts 2 from it (so 3 becomes 1, 6 becomes 4, etc...), and then uses that number as the Month, still hard coding the day of "1", and appends the current year.

This reduced the total number of lines down to 3. You could go a step further and try to cram it all on one line like the following, but please don't do that as it is a lot harder to read / unnecessarily uses two cmdlet calls for the same data / we aren't paying by the carriage return :

[code language="powershell"]
$StartDate = "$((([Math]::Ceiling((Get-Date).Month / 3)) * 3 - 2))/1/" + (Get-Date).Year.ToString()
[/code]

Lastly I provided my colleague this additional code in case the maximum the query could go backwards in time was 90 days, so that the StartDate would be 90 days or the first day of the current quarter, which ever was more recent/closest to today:

[code language="powershell"]
$90Days = (Get-Date).AddDays(-90)
If ((Get-Date $StartDate) -lt $90Days) {
$StartDate = $90Days.ToShortDateString()
}
[/code]

So there you have it, a couple of lines to get the first day of the current quarter in PowerShell.

Thanks!

Dan Sheehan

Senior Premier Field Engineer