Gets the next date

Get-TaskStartDate.ps1

function Get-TaskStartDate {
    <#
    .SYNOPSIS
        Get a start date from a string expression.

    .DESCRIPTION
        Finds the start date from a string expression.

    .EXAMPLE
        Get-TaskStartDate -Day '1st Monday'

        Gets the next occurrence of the first monday of a month.
    
    .EXAMPLE
        Get-TaskStartDate -Day Tuesday

        Gets the next tuesday in the month.
    
    .EXAMPLE
        Get-TaskStartDate -Day '2nd tuesday' -Offset 2

        Gets a date two days after the 2nd tuesday of the month.

    .EXAMPLE
        Get-TaskStartDate -Day 'last fri'

        Gets the last friday of the month.
    #>

    [CmdletBinding()]
    param (
        # The Day may be an expression, such as 3rd Wednesday, or a day of the week such as Tuesday.
        [string]$Day,

        # Apply an offset (in days) to the start date.
        [int]$Offset
    )

    function Get-DayInMonth {
        [CmdletBinding()]
        param (
            [Parameter(Mandatory)]
            [DayOfWeek]$Day,

            # The starting point.
            [DateTime]$Date = (Get-Date -Day 1),

            # Week number
            [int]$Week = 1,

            # Apply an offset (in days) to the start date.
            [int]$Offset,

            [switch]$Backwards
        )

        while ($Date.DayOfWeek -ne $Day) {
            $Date = $Date.AddDays(1 * (1, -1)[$Backwards.IsPresent])
        }

        $Date.AddDays(7 * ($Week - 1) + $Offset)
    }

    $today = Get-Date

    $offSetParam = @{
        Offset = $Offset
    }

    switch -regex ($Day) {
        { $Day -as [DayOfWeek] } {
            $start = $today
            do {
                $date = Get-DayInMonth -Day $Day -Date $start @offSetParam
                $start = $start.AddDays(1)
            } until ($date -gt $today)
            break
        }
        '^\d+$' { 
            $date = Get-Date -Day $Day
            if ($date -lt $today) {
                $date = $date.AddMonths(1)
            }
            break
        }
        '^(?<week>\d+)\S* +(?<day>[a-z]+)$' {
            if (-not ($matches['day'] -as [DayofWeek])) {
                throw 'Unexpected day format'
            }

            $matches.Remove(0)
            $date = Get-DayInMonth @matches -Offset $Offset
            if ($date -lt $today) {
                $date = Get-DayInMonth @matches -Date (Get-Date -Day 1).AddMonths(1) @offSetParam
            }
            break
        }
        '(?<modifier>\w+) +(?<day>[a-z]+)$' {
            switch ($matches['modifier']) {
                'first'  { Get-TaskStartDate ("1st {0}" -f $matches['day']) }
                'second' { Get-TaskStartDate ("2nd {0}" -f $matches['day']) }
                'third'  { Get-TaskStartDate ("3rd {0}" -f $matches['day']) }
                'forth'  { Get-TaskStartDate ("4th {0}" -f $matches['day']) }
                'last'   {
                    $startDate = (Get-Date -Day 1).AddMonths(1).AddDays(-1)
                    $date = Get-DayInMonth -Day $matches['day'] -Date $startDate -Backwards @offSetParam
                    
                    if ($date -lt $today) {
                        $date = Get-DayInMonth -Day $matches['day'] -Date $startDate.AddMonths(1) @offSetParam
                    }
                }
            }
        }
        default {
            throw 'Invalid date expression'
        }
    }

    $date.Date
}