Below is the formula to Calculate Start Date and End Date of current month

Month Start Date

UpdateContext({
   monthStartDate: 
   Date(
     Year(Now()), Month(Now()), 1)
   )
})

---

Month End Date

UpdateContext({
   monthEndDate:
   DateAdd(
     DateAdd(
        Date(Year(Now()), Month(Now()),1),
        1, Months
     ),
     -1, Days
   )
})