@Sarohadesh also the ISO formatting variations of the DateTimeFormat() function were introduced in a more recent version:
CHANGES FOR VERSION 6.15.0 (as compared to 6.14.0)
AFL: new modes added to DateTimeToStr() that use ISO regardless of regional settings mode = 3 (iso date and time YYYY-MM-DD HH:MM:SS) HH:MM:SS part is only included for non-EOD records, 4 (iso date only YYYY-MM-DD), 5 (iso time only HH:MM:SS)
So, to use these ISO modes (and a ton of other great features) you need to upgrade! (By the way, this forum supports only versions >= 6.0).
In any case, you can try to use the function without any parameter and the split the resulting string using some of the many AFL string manipulations functions.