DATEADD in EU format issue

I have the day of payment (PayDay value).
and that the next payment will be after 1 month.

I need formula which generates date of next payment,
In correlation based with today, I have to pay either this month or next (the formula decides)

there are some peculiarities
If the payment date of the current month has already passed, PayDayToPayDate is before today, then add a period of +1 month to the date. OK

I solved my task with US Date Format and it works well no bugs there.
But EU is bugged - it increases value of month instead of day value.
Am I making a mistake in the formula?

My code here

US_PayDayToPayDate

IF(PayDay,DATETIME_FORMAT(DATETIME_PARSE(MONTH(NOW()) & "/" & PayDay & "/" & YEAR(NOW()), "L"), "L"),"")

US_NextPayment

IF(US_PayDayToPayDate,IF(US_PayDayToPayDate < DATETIME_FORMAT(NOW(), "L"), DATETIME_FORMAT(DATEADD(US_PayDayToPayDate, 1, 'months'), "L"), US_PayDayToPayDate),"")

EU_PayDayToPayDate

IF(PayDay,DATETIME_FORMAT(DATETIME_PARSE(PayDay & "." & MONTH(NOW()) & "." & YEAR(NOW()), "DD.MM.YYYY"), "DD.MM.YYYY"),"")

EU_NextPayment

IF(EU_PayDayToPayDate,IF(EU_PayDayToPayDate < DATETIME_FORMAT(NOW(), "DD.MM.YYYY"), DATETIME_FORMAT(DATEADD(EU_PayDayToPayDate, 1, 'months'), "DD.MM.YYYY"), EU_PayDayToPayDate),"")

JustForTest

DATETIME_FORMAT(DATEADD(EU_PayDayToPayDate, 1, 'months'), "DD.MM.YYYY")