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.
        [Parameter(ValueFromPipeline)]
        [string]$Day,

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

        # Use a specific culture to parse the string.
        [System.Globalization.CultureInfo]$Culture = (Get-Culture)
    )

    begin {
        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)
        }

        function Convert-DayName {
            [CmdletBinding()]
            param (
                [Parameter(Mandatory)]
                [string]$Day,

                [Parameter(Mandatory)]
                [string[]]$DayNames,

                [switch]$Try
            )

            $Day = $Day.ToLower()
            $index = $DayNames.IndexOf($Day)

            if ($index -eq -1) {
                if ($Try) {
                    return
                } else {
                    throw 'Unexpected day format'
                }
            }

            if ($index -ge 14) {
                $index -= 14
            } elseif ($index -ge 7) {
                $index -= 7
            }
            $index -as [DayOfWeek]
        }

        $today = [DateTime]::Today

        $offSetParam = @{
            Offset = $Offset
        }

        $dateFormat = $Culture.DateTimeFormat
        $dayNames = @(
            $dateFormat.DayNames
            $dateFormat.AbbreviatedDayNames
            $dateFormat.ShortestDayNames
        ) | ForEach-Object ToLower
    }

    process {
        try {
            $convertedDay = Convert-DayName -Day $Day -DayNames $dayNames -Try
            switch -regex ($Day) {
                { $convertedDay -as [string] } {
                    $start = $today
                    do {
                        $date = Get-DayInMonth -Day $convertedDay -Date $start @offSetParam
                        $start = $start.AddDays(1)
                    } until ($date -ge $today)
                    break
                }
                '^\d+$' { 
                    $date = Get-Date -Day $Day
                    if ($date -lt $today) {
                        $date = $date.AddMonths(1)
                    }
                    break
                }
                '^(?<week>\d+)\S* +(?<day>[a-z]+)$' {
                    $matches['day'] = Convert-DayName -Day $matches['day'] -DayNames $dayNames

                    $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
        } catch {
            Write-Error -ErrorRecord $_
        }
    }
}