This is the recommended expression for rounding dates to the beginning of the next month:

BOM1_DT:= #NEXTBEGMTH (A_DT + 1) &
; add one day to every date to take advantage of coincident or following

or 

BOM1_DT:= 1 #MONTHROUND (A_DT + 1) &
; add one day to every date to take advantage of coincident or following

A_DT may be either a single date (e.g., 7/16/2015, DOH, #DODEC) or an array of dates (e.g., #DATE).   

Why are we adding 1 day to A_DT before we round the date(s)? Because #NEXTBEGMTH (and #MONTHROUND) arounds to the coincident or following beginning of month. By adding 1 day, 1/1/2020 becomes 1/2/2020, and #NEXTBEGMTH 1/2/2020 returns 2/1/2020, the beginning of the next month. If A_DT is 1/31/2020, then adding 1 day gives us 2/1/2020, which is the beginning of the next month, so #NEXTBEGMTH 2/1/2020 returns 2/1/2020 (i.e., coincident or following), the beginning of the next month for 1/31/2020.