// first day of month (2021-01-01 00:00:00)
Carbon::now()->firstOfMonth()->startOfDay()
 
// first day of month in timestamp (1609459200)
Carbon::now()->firstOfMonth()->startOfDay()->timestamp
 
// last day of month (2021-01-31 23:59:59)
Carbon::now()->lastOfMonth()->endOfDay()
 
// last day of month in timestamp (1640995199)
Carbon::now()->lastOfMonth()->endOfDay()->timestamp