Get the last day of a month on powershell - powershell

Question

I'm trying to put in a .txt file the first day and the last day of the months using PowerShell.
In the exemple below i was trying to get the first and the last day of July, but i'm just getting the first day. The second line of the script isn't working.
#PowerShell "(Get-Date).AddMonths(1).ToString('01/MM/yyyy')" >>dates.txt
$LastDayInMonthString = "$($(get-date).AddMonths(1).ToString("dd/mm/yyy"))$LastDayInMonth" >>dates.txt
Someone can say me what is wrong?
I wanted a .txt file like it: 01/07/2018, 31/07/2018.
The first line write the first day of next month,
and second line write the last day of that month.

Answer 1

Much simpler solution is to call into the DaysInMonth function
[DateTime]::DaysInMonth(2018, 11)
For the current month that would look like:
$today = get-date
$lastDay = [DateTime]::DaysInMonth($today.Year, $today.Month)
$firstDate = [DateTime]::new($today.Year, $today.Month, 1)
$lastDate = [DateTime]::new($today.Year, $today.Month, $lastDay)
$firstDate
$lastDate
This also works around any hindering daylight savings changes and other weird things that can happen with timezones etc.
Or if pure strings are all you need:
(get-date -Format "yyyy/MM") + "/1"
(get-date -Format "yyyy/MM") + "/" + [DateTime]::DaysInMonth((get-date).Year, (get-date).Month)

Answer 2

An easy way is to take the last day of the previous year and add 1..12 months to it:
1..12 | % { (New-Object DateTime(2017,12,31)).AddMonths($_) }
Output will be in the user's date/time format, in my case Dutch:
woensdag 31 januari 2018 00:00:00
woensdag 28 februari 2018 00:00:00
zaterdag 31 maart 2018 00:00:00
maandag 30 april 2018 00:00:00
donderdag 31 mei 2018 00:00:00
zaterdag 30 juni 2018 00:00:00
dinsdag 31 juli 2018 00:00:00
vrijdag 31 augustus 2018 00:00:00
zondag 30 september 2018 00:00:00
woensdag 31 oktober 2018 00:00:00
vrijdag 30 november 2018 00:00:00
maandag 31 december 2018 00:00:00
If required you can format it as you need it, e.g.
1..12 | % { (New-Object DateTime(2017,12,31)).AddMonths($_).ToString("yyyyMMdd") }
20180131
20180228
20180331
20180430
20180531
20180630
20180731
20180831
20180930
20181031
20181130
20181231

Answer 3

This seems simple enough
$firstDate = [DateTime]::new($reportDate.Year, $reportDate.Month, 1)
$lastDate=$firstDate.AddMonths(1).AddDays(-1)

Answer 4

Edit removed the for only date unneccessary time adjustments
In PowerShell to get the first day and last of next month
$CIGB = New-Object System.Globalization.CultureInfo('en-GB')
'{0}, {1}' -f (Get-Date -Day 1).AddMonths(1).ToString('d',$CIGB),
(Get-Date -Day 1).AddMonths(2).AddDays(-1).ToString('d',$CIGB)|sc dates.txt
The $CIGB is neccessary for me because my local date separator overrides the /
If your short date format 'd' returns dd/MM/yyyy the first line and the ,$CIGB can be removed.
01/07/2018, 31/07/2018
This can be wrapped in a single (albeit quite long) line.
powershell -nop -c "$CIGB=New-Object System.Globalization.CultureInfo('en-GB');'{0}, {1}' -f (Get-Date -Day 1).AddMonths(1).ToString('d',$CIGB),(Get-Date -Day 1).AddMonths(2).AddDays(-1).ToString('d',$CIGB)">>dates.txt
> type dates.txt
01/07/2018, 31/07/2018

Answer 5

Strings can be passed to Get-Date. The -Format parameter also accepts strings/variables.
# Get the previous month
$LastMonth = (Get-Date).AddMonths(-1)
# Format the first day of the month
$FirstDayOfMonth = $LastMonth | Get-Date -Format "yyyy/MM/01"
# Calculate the number of days (last day) in the month
$LastDay = [DateTime]::DaysInMonth($LastMonth.Year, $LastMonth.Month)
# Use it in the format parameter
$LastDayOfMonth = $LastMonth | Get-Date -Format "yyyy/MM/$lastDay"