START() fn is used to return first date in time period. Which function is used to return first month of time period?
Make the line-item to 'Month' format. Then say PERIOD(START())
If you want to find first Period of the timescale => OFFSET(1,-1,0) = 0
If you want to get the first month of every year, there are multiple ways to do so. You can convert the above boolean into number and then add 12 into it and then find MOD of that number. Result of 1 is the First month of every year.