PowerShell Code Snippets

$now = $( Get-Date )  # or [DateTime]::Now

$calendarWeekRule = 2 # first 4-day week, see https://docs.microsoft.com/en-us/dotnet/api/system.globalization.calendarweekrule

$firstDayOfWeek   = 1 # Monday, see https://docs.microsoft.com/en-us/dotnet/api/system.dayofweek

[System.Globalization.DateTimeFormatInfo]::CurrentInfo.Calendar.GetWeekOfYear( $now, $calendarWeekRule, $firstDayOfWeek )

$nextmonth = ( Get-Date -Day 1 ).AddMonths( 1 ).Date

$today = ( Get-Date ).Date

"{0} days left this month (including today)" -f ( $nextmonth - $today ).Days

$year = ( Get-Date ).Year

$leapyear = [DateTime]::IsLeapYear( $year )

# or by checking the number of days in a year, tip from Joe Caverly:

$leapyear = ( ( [DateTime] "$year-12-31" ).DayOfYear -eq 366 )

# ditto, but without requiring variable $year:

$leapyear = ( ( Get-Date -Month 12 -Day 31 ).DayOfYear -eq 366 )

# or by checking if February has 29 days:

$leapyear = ( [DateTime]::DaysInMonth( $year, 2 ) -eq 29 )

# ditto, but without requiring variable $year:

$leapyear = ( ( Get-Date -Month 3 -Day 1 ).AddDays( -1 ).Day -eq 29 )

# or, the hard way:

[bool]$leapyear = ( [bool]!( $year % 4 ) -and [bool]( $year % 100 ) ) -or [bool]!( $year % 400 )

$today = ( Get-Date )

$year = $today.Year

if ( ( $today.Month -eq 12 ) -and ( $today.Day -ge 25 ) ) {

	$year += 1 # add 1 year if we are past Christmas this year

}

$christmas = ( Get-Date "$year-12-25" )

$wait = ( New-TimeSpan -Start $today -End $christmas )

Write-Host ( "Please be patient for another {0} day(s)" -f $wait.Days )

# Easter is at first Sunday after the first full moon at or after the Spring equinox (21 March)

# Calculation explained: https://www.timeanddate.com/calendar/determining-easter-date.html

Set-Variable -Name ReferenceFullMoon -Option Constant -Value $( Get-Date -Date '2022-01-17 23:48' ) -ErrorAction Ignore

Set-Variable -Name MoonCycleDays     -Option Constant -Value 29.53                                  -ErrorAction Ignore

if ( $args.Length -eq 1 ) {

	$year = $args[0] # enclose year in quotes if less than 100, and always use 4 digits, e.g. '0033'

} else {

	$year = $( Get-Date ).Year # or specify any other year on the command line

}

# For Easter calculations, the Spring equinox is always assumed to be at 21 March

$springequinox = $( Get-Date -Date "$year-03-21" ) # 21 March

# Calculate full moon cycles to first full moon after Spring equinox of specified year

$fullcycles = [Math]::Ceiling( ( New-TimeSpan -Start $ReferenceFullMoon -End $springequinox ).Days / $MoonCycleDays )

# Date of first full moon after Spring equinox of specified year

$fullmoonafterequinox = $ReferenceFullMoon.AddDays( $fullcycles * $MoonCycleDays )

# First Sunday following first full moon at or after Spring equinox of specified year

$eastersunday = $fullmoonafterequinox.AddDays( 7 - [int]$fullmoonafterequinox.DayOfWeek ).Date

# Display the result

if ( ( New-TimeSpan -Start ( Get-Date ).Date -End $eastersunday ).Days -lt 0 ) {

	"Easter {0} was at {1}" -f $year, $eastersunday.ToLongDateString( )

} else {

	"Easter {0} will be at {1}" -f $year, $eastersunday.ToLongDateString( )

}

# Code to convert today's (Gregorian) date to Julian date by David Yaw: https://stackoverflow.com/a/5254812

$juliandate = ( Get-Date ).Date.ToOADate( ) + 2415018.5

# or (see https://en.wikipedia.org/wiki/Julian_day#Variants for details):

$juliandate = ( Get-Date ).Date.Ticks / 864000000000 + 1721425.5

# Convert Julian date back to Gregorian date:

$gregoriandate = [DateTime]::FromOADate( $juliandate - 2415018.5 )

# or:

$gregoriandate = $( Get-Date -Date '0001-01-01' ).AddTicks( ( $juliandate - 1721425.5 ) * 864000000000 )

# Demo: convert between Gregorian and Julian dates:

$todaygregorian = $( Get-Date ).Date

"Gregorian date for today    : {0}" -f $todaygregorian.ToString( 'yyyy-MM-dd' )

$juliandate = $todaygregorian.ToOADate( ) + 2415018.5 # or: $todaygregorian.Ticks / 864000000000 + 1721425.5

"Converted to Julian date    : $juliandate"

$gregoriandate = [DateTime]::FromOADate( $juliandate - 2415018.5 ) # or: $( Get-Date -Date 0001-01-01 ).AddTicks( ( $juliandate - 1721425.5 ) * 864000000000 )

"Converted back to Gregorian : {0}" -f $gregoriandate.ToString( 'yyyy-MM-dd' )