Getting Yesterday’s Date

@DoctorDNS

Q: How can I get yesterday’s date?

A: You can use a combination of the Get-Date cmdlet and .NET Time/Date methods.

First, let’s look at dates in PowerShell and .NET Then we can look at how to calculate yesterday and use that in your scripts.

Dates in PowerShell

Let’s start by looking at how you can deal with dates and times. As you probably know, PowerShell contains the Get-Date cmdlet. This cmdlet returns a .NET System.DateTime object.

Using the Get-Date cmdlet, you can get any date and time, and either display it or store it in a variable. To get today’s date. you could do this:

PS C:> # Get the current date
PS C:> Get-Date
08 January 2021 11:24:46

# Store the date in a variable
$Now = Get-Date
$Now
08 January 2021 11:24:47

As mentioned, the Get-Date cmdlet returns an object whose type is System.DateTime. This .NET structure provides a rich set of properties and methods to help you manipulate the date/time object. See the System.DateTime documentation for more details on this structure. A date and time object contains both a date and a time. This means you can create an object with just a date or just a time, or both, which gives you huge flexibility in handling dates and times.

If you run Get-Date and specify no parameters, the cmdlet returns the current date and time. There are several parameters you can specify that allow you to create an object for a particular date, like this:

PS C:> # Using the -Date Parameter and a date string
PS C:> Get-Date -Date '1 August 1942'
01 August 1942 00:00:00

# Using the -Month, Day, Year to be specific and avoid parsing
PS C:> Get-Date -Month 8 -Day 1 -Year 1942 -Hour 0 -Minute 0 -Second 0
01 August 1942 00:00:00

You can see the other features of Get-Date to help get the date in the exact format you need, see the Get-Date help information.

Obtaining Yesterday’s Date

So as you can see, you can use Get-Date to return a specific date/time. So how do you get yesterday’s date – or the date or last month or last year? The trick here is to use the object returned from Get-Date. The object has a type of System.DateTime which contains a number of methods allowing you to add increments of time – a month, a day, etc to the object.

To get yesterday’s date (or tomorrow’s) you create a date and time object for today using Get-Date with no parameters. Then you use the AddDays() method to add/subtract some number of days, like this:

PS C:> # Get today's Date
PS C:> $Today     = Get-Date
PS C:> $Yesterday = $Today.AddDays(-1)
PS C:> $Yesterday
19 February 2021 12:13:51

PS C:> # Or more simply
PS C:> $Yesterday = (Get-Date).AddDays(-1)
PS C:> $Yesterday
19 February 2021 12:13:52

PS C:> # Get tomorrow's date
PS C:> $Tomorrow  = (Get-Date).AddDays(1)
PS C:> $Tomorrow
21 February 2021 12:13:54

It is worth noting that a System.DateTime object is immutable. This means you can not change property values after you create the object. If you use any of the Add methods, .NET returns a new object with updated property values.

Using Yesterday’s Date

There are a variety use cases for getting a date in the past (or the future), including:

• Identifying files that are older/younger than a day/month/etc ago
• Determining which AD Users have not logged on in the last week
• Creating a file name for a file representing last weeks information.

Here are some examples:

PS C:> # Finding files newer than yesterday
PS C:> $Yesterday = (Get-Date).AddDays(-1)
PS C:> Get-ChildItem | Where-Object LastAccessTime -gt $Yesterday

    Directory: C:

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a---          20/02/2021    14:20          11041 GratefulDead Show List.txt

PS C:> # Getting users who have logged on in the past day
PS C:> Get-ADUser -Filter * -Property LastLogonDate | Where-Object LastlogonDate -gt $Yesterday

DistinguishedName : CN=Administrator,CN=Users,DC=cookham,DC=net
Enabled           : True
GivenName         : Jerry
LastLogonDate     : 20/02/2021 04:20:42
Name              : Jerry Garcia
ObjectClass       : user
ObjectGUID        : ae31ca0d-3f01-4eb4-8593-b1d79c71f912
SamAccountName    : JerryG
SID               : S-1-5-21-2550804810-443649076-1856842782-500
Surname           : Garcia

# Creating a file with yesterday's date
PS C:> # Creating a file with today's date
PS C:> $Yesterday     = (Get-Date).AddDays(-1).ToString() -replace '/','-'
PS C:> $YesterdayDate = ($Yesterday -split ' ')[0]
PS C:> $YesterdayFN   = "Results for $YesterdayDate.Txt"
PS C:> 
PS C:> New-Item -Path C:Results -Name  $YesterdayFN -ItemType File

Directory: C:Results

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a---          20/02/2021    12:56              0 Results for 19-02-2021.Txt

In that last example, you need to do a bit of manipulation of the date/time returned by Get-Date in order to get a filename that Windows accepts. This manipulation is needed because Get-Date returns a string that contains the “/” character New-Item views as a path character. You use the -Replace operator to replace the “/” character with a “-“. Additionally, after performing the replacement, you end up with an (unneeded) time value. You can use the -Split operator to pull out just the date, which is what you want for the file name. Once you do get the date, you can create you can create a file name for the file.

Another way to generate the file name based on Get-Date would be to use the ToString() method and specify the exact output you want, like this:

$YesterdayDate = (Get-Date).AddDays(-1).ToString('yyyy-MM-dd')         
$YesterdayFN   = "Results for $YesterdayDate.Txt"

Another point worth making is that Windows tries to display dates in a culture-aware way. Get-Date does a fairly good job in most cases of converting a date string into the date you wanted. But if you want a specific result, using ToString() and a date format string is possibly better – and fewer lines of code.

Needless to say, you could do all those file name manipulations operations as a one-liner. I leave that as an exercise for you!

Summary

.NET provides a rich date and time structure (System.DateTime). This structure contains a number of properties such the day, month, hour, millisecond for a given date/time. You also get a wide range of methods that enable you to manipulate dates by adding or subtracting hours, days, etc. You can use Get-Date cmdlet to get the current date/time or an object for a specific date/time. Get-Date returns an object of System.DateTime. You use the methods of the System.DateTime structure to get relative dates, such as yesterday, last month or 2 years 42 days, and 32 milliseconds.

Tip of the Hat

This article is based on an earlier Scripting Guys blog article at How can I get yesterday’s date?. Not sure who wrote it.

@DoctorDNS PowerShell Evangelist, PowerShell Community Blog

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