try this out:
CurrentDateTime = DateTime.GetCurrentDateTime.Local DateTimeFormat: DateTime.DateTimeFormat.DateAndTime
FormattedDateTime = Text.ConvertDateTimeToText.FromCustomDateTime DateTime: CurrentDateTime CustomFormat: "dd"
TextAsNumber = Text.ToNumber Text: FormattedDateTime
FirstDate = DateTime.Add DateTime: CurrentDateTime TimeToAdd: -TextAsNumber + 1 TimeUnit: DateTime.TimeUnit.Days
LastDate = DateTime.Add DateTime: FirstDate TimeToAdd: 1 TimeUnit: DateTime.TimeUnit.Months
LastDayInMonth = DateTime.Add DateTime: LastDate TimeToAdd: -1 TimeUnit: DateTime.TimeUnit.Days
Display.ShowMessageDialog.ShowMessage Message: "First date of Month : %FirstDate% \nLast date of month : %LastDayInMonth%"