New Post - Get Yesterday's Date

Posts/2021/02/ScriptingGuy.0002.GetDateOfYesterday.md

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+---
+post_title: Getting Yesterday's Date
+username: tfl@psp.co.uk
+Catagories: PowerShell
+tags: .NET, Scripting Guys Update
+Summary: Shows how to get a recent date and use that in your scripting.
+---
+
+
+**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:
+
+```powershell-console
+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](https://docs.microsoft.com/dotnet/api/system.datetime) 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:
+
+```powershell-console
+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](https://docs.microsoft.com/powershell/module/microsoft.powershell.utility/get-date?view=powershell-7.1).
+
+## 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:
+
+```powershell-console
+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:
+
+```powershell-console
+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  $YesterdayFN -ItemType File
+
+Directory: C:\Results
+
+Mode                 LastWriteTime         Length Name
+----                 -------------         ------ ----
+-a---          20/02/2021    12:56              0 Results for 19-02-2021.Txt
+```
+
+In that last example, you need to do a bit of manipulation of the date/time returned by `Get-Date` in order to get a filename that Windows accepts.
+This manipulation is needed because `Get-Date` returns a string that contains the "/" character `New-Item` views as a path character.
+You use the `-Replace` operator to replace the "/" character with a "-".
+Additionally, after performing the replacement, you end up with an (unneeded) time value.
+You can use the `-Split` operator to pull out just the date, which is what you want for the file name.
+Once you do get the date, you can create you can create a file name for the file.
+
+Another way to generate the file name based on `Get-Date` would be to use the **ToString()** method and specify the exact output you want, like this:
+
+```powershell
+$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?](https://devblogs.microsoft.com/scripting/how-can-i-get-yesterdays-date).
+Not sure who wrote it.