Get First/Last Day of Month from DateTime

Hello,

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