Getting relative dates (like yesterday or one week ahead) is easy once you know the Add…() methods every DateTime object supports. This would give you yesterday:
$today = Get-Date $yesterday = $today.AddDays(-1) $yesterday
$yesterday will be exactly 24 hours before now. So what if you would like yesterday, but at a given time? Let's say, yesterday at midnight?
If you really want today at midnight, things are easy:
$todayMidnight = Get-Date -Hour 0 -Minute 0 -Second 0 $todayMidnight
And if you want another date with a given time, use Get-Date again to override the parts of the date you need. This is yesterday at midnight:
$today = Get-Date $yesterday = $today.AddDays(-1) $yesterday | Get-Date -Hour 0 -Minute 0 -Second 0