Convert-StarDate.ps1

## Convert-StarDate.ps1
#Requires -Version 3
<#
    .SYNOPSIS
    Calculates the stardate.

    .DESCRIPTION
    Returns the current stardate with no parameters, or you can enter a specific date as a string or as a DateTime `
    object and Get-Stardate will return the stardate for that time.

    .PARAMETER DateTime
    The date and/or time you want to calculate the stardate for. Defaults to the current time. This may be entered `
    either as a DateTime object or as a string.

    .PARAMETER Decimal
    A number specifying how many decimal places you want to carry the calculation to.
    
    .EXAMPLE
    Get-Stardate
    Returns the current stardate.

    .EXAMPLE
    Get-Stardate "4/7/2014 10:20 am"
    Returns the stardate at that time in the current time zone.

    .EXAMPLE
    Get-Stardate -Decimal 2
    Returns the current stardate with two decimal points of precision.

    .EXAMPLE
    Get-Stardate -Stardate 15814.2
    Returns the datetime of that particular stardate.

    .EXAMPLE
    (Captains-Log -Stardate (Get-Stardate)).NewEntry('Meme us up scotty')
    Example being used in another function.

    .INPUTS
    You can pipe in either a DateTime object or a string that can be converted into a DateTime object. Input is `
    optional; with no input the current date is used.

    .OUTPUTS
    A string representing a formatted stardate number. A string is returned instead of a double so that `
    stardates ending in ".0" do not have the decimal lopped off.

    .NOTES
    Cast the output to [double] if you need to do math with it.
#>
function Convert-StarDate {
    [CmdletBinding(DefaultParameterSetName="ConvertToStardate")]
    [OutputType("ConvertToStardate", [string])] # cast to double if math'ing the outhput
    [OutputType("ConvertFromStardate", [DateTime])]

    param
    (
        [Parameter(Mandatory=$false,
            Position=0,
            ParameterSetName="ConvertToStardate",
            ValueFromPipeline=$true)]
            [ValidateScript({$_ -gt [DateTime] "1/1/2000Z"})]
            [Alias("To")]
            [DateTime] $DateTime,

        [Parameter (Mandatory=$false,
            ParameterSetName = "ConvertToStardate")]
            [ValidateRange (0, 10)]
            [Decimal] $Decimal,

        [Parameter (Mandatory=$false,
            ParameterSetName="ConvertFromStardate",
            ValueFromPipeline=$true)]
            [ValidateScript({$_ -ge 0})]
            [Alias("From")]
            [double] $Stardate
    )

    switch ($PSCmdlet.ParameterSetName)
    {
        "ConvertToStardate"
        {
            $DateTime = try{if(!$Datetime){throw}}catch{Get-Date}
            $Decimal  = try{if(!$Decimal){throw}}catch{[Decimal]"1"}
            $Result   = ($DateTime - (Get-Date "1/1/2000Z")).TotalDays / 0.36525
            "{0:F$Decimal}" -f $Result
        }
        "ConvertFromStardate"
        {
            $TotalDays = (Get-Date "1/1/2000Z").TotalDays + $Stardate * 0.36525
            $TotalDays = [TimeSpan]::FromDays($TotalDays)
            (Get-Date "1/1/2000Z") + $TotalDays
        }
    }
}

if ($MyInvocation.ExpectingInput)
{
    $input | Convert-StarDate @args
}
elseif (!($MyInvocation.InvocationName -eq '.' -or $MyInvocation.Line -eq ''))
{
    Convert-StarDate @args
}