It is forward counting on monthly basis, you are free to put in or take out however much you want
example:

28 Nov RM1,000 will only be counted for pro-rated 1/12 * dividend let's say 6% if you maintain it until 1st January, if you withdraw it by 31st Dec then it will be zero interest earned. If you keep until ex-dividend date of 30th Sept 2019 and only withdraw on 1st October 2019 then you will get the interests counted for month of Dec until Sep which is 10/12 * 6%

basically our money needs to FULLY sit in untouched per calendar month, not per 30 days basis. So it doesn't make a difference if you put your money in on 1st or 30th/31st of that month.
as above mentioned, let's take a simulation using ASM3 at 6% for example:

1st Sep RM4,000 IN
15th Sep RM5,000 IN
End Sep Balance: RM9,000

1st Oct RM5,000 IN
20th Oct RM3,000 OUT
31st Oct RM8,000 IN
End Oct Balance: RM15,000

1st Nov RM2,000 OUT
End Nov Balance: RM13,000

31st Dec RM3,000 OUT
End Dec Balance: RM10,000

Assume you stop putting in/taking out until following year 1st Oct then your dividend shall be calculated as follow:

Sep- RM0 earned (not full month)
Oct- Balance>Sep so take lower RM9,000 x 1/12 x 6% = RM45
Nov- Balance<Oct so take lower RM13,000 x 1/12 x 6% = RM65
Dec- Balance<Nov so take lower RM10,000 x 1/12 x 6% = RM50
Jan- Sep Balance maintain RM10,000 x 9/12 x 6% = RM450

Total dividend earned: RM610

Anyone sifus can verify the above? Much appreciated since I just joined, no historical data to counter-check