Get First/Last Day of Month from DateTime


I would like to populate one column with the first day of the month (eg 06/01/2021) and a separate column with the last day of the month (eg 06/30/2021). I tried the following to get the first day from a Datetime Type but get an error:

`month(this) + "/" + "1" + year(this)`

Error: error: String can’t be coerced into Fixnum

What’s the correct way to get these values? Thanks

I first made a formula field for my test date
TestDate = date(‘12/1/2021’)

Then two Formula Fields
First_Of_Month Formula:= field(“TestDate”) - days(day(field(“TestDate”)) - 1)
End_Of_Month Formula:= (field(“First_Of_Month”) + month(1)) - days(1)