How to set datetime value in Pega

In Pega, if we have to set date, it picks only a specific format (yyyymmddThhmmss.SSS GMT)

Context: 

LastUpdateDateTime is a property of type "DateTime", and we need to do property-set from activity with a value 1989-09-18, Pega accepts only a specific format (yyyymmddThhmmss.SSS GMT) in Pega without hyphens(-) or slash (/)

yyyy	year
mm	month
dd	day
T	Delimiter
hh	hour
mm	month
ss	seonds
.	Dot
SSS	Micro Seconds
" "	Space
GMT	GMT Timezone

Answer:

LastUpdateDateTime = 19890918T053347.789 GMT(Correct)
LastUpdateDateTime = 1989-09-18T053347.789 GMT  (InCorrect)
LastUpdateDateTime = 18/09/1989T053347.789 GMT  (InCorrect)