(Get-Date $date.Date -day 1).AddMonths(1).AddDays(-1)
PS C:\> $Date = Get-Date >> (Get-Date $date.Date -day 1).AddMonths(1).AddDays(-1) Sunday, March 31, 2019 12:00:00 AM
(Get-Date $date.Date -day 1).AddMonths(1).AddDays(-1)
ExamplePS C:\> $Date = Get-Date >> (Get-Date $date.Date -day 1).AddMonths(1).AddDays(-1) Sunday, March 31, 2019 12:00:00 AM