AIRAC dates generator - enumerates AIRAC cycles: previous, current and next.

Out-AIRACCycles.ps1

Function Out-AIRACCycles
{
    Param ( [DateTime] $currentDay = ( Get-Date )
          , [DateTime] $initialDay = ( Get-Date -Year 2015 -Month 1 -Day 8 )
          ,            $interval   = 28
          ,            $culture    = [System.Globalization.CultureInfo]::GetCultureInfo("en-US") )

    
    $offset = (New-TimeSpan -Start $initialDay -End ( Get-Date -Year ( $currentDay.Year - 1 ) -Month 1 -Day 1 ) ).Days
    $offset .. ( $offset + 365 * 3 ) |
        ? { $_ % 28 -eq 0 } |
        select @{ N="date"; E={ $initialDay.AddDays( $_ ) } } |
        select -expand date |
        group Year |
        % {
            $year  = $_.Name
            $dates = $_.Group            
            1 .. $_.Count | select @{ N="Cycle";         E={ $_ } },
                                   @{ N="Ident";         E={ "$($year.Substring(2,2))$($_.ToString().PadLeft(2, '0' ) )" } },
                                   @{ N="EffectiveDate"; E={ Get-Date $dates[ $_-1 ] -Format d } },
                                   @{ N="AIRAC";         E={ $dates[ $_-1 ].ToString("dd MMM yy", $culture).ToUpperInvariant() } },
                                   @{ N="distance";      E={ ( New-TimeSpan -Start $currentDay -End $dates[ $_-1 ] ).Days } }
        } |
        ? { [Math]::Abs( $_.distance ) -lt $interval * 2 } |
        select Cycle, Ident, EffectiveDate, AIRAC -First 3
}

Usage samples

No arguments, no pipes. It outputs AIRAC cycles for today.

PS >Out-AIRACCycles

Cycle Ident EffectiveDate AIRAC    
----- ----- ------------- -----    
    3 1703  02.03.2017    02 MAR 17
    4 1704  30.03.2017    30 MAR 17
    5 1705  27.04.2017    27 APR 17

Specific date (i.e.: 5th january 2020):

PS >Out-AIRACCycles -currentDay ( Get-Date -Year 2020 -Month 1 -Day 5 )

Cycle Ident EffectiveDate AIRAC    
----- ----- ------------- -----    
   13 1913  05.12.2019    05 DEC 19
    1 2001  02.01.2020    02 JAN 20
    2 2002  30.01.2020    30 JAN 20

Only current cycle:

PS >Out-AIRACCycles | select -Index 1

Cycle Ident EffectiveDate AIRAC    
----- ----- ------------- -----    
    4 1704  30.03.2017    30 MAR 17

Only next cycle:

PS >Out-AIRACCycles | select -Index 2

Cycle Ident EffectiveDate AIRAC    
----- ----- ------------- -----    
    5 1705  27.04.2017    27 APR 17