Expressions node behaves not as expected

Dear KNIMErs,

I am playing around with the Expressions node. I want to nest 2 formulas inside each other. One replace formula, that overwrites basically a timestamp. Than - outside of this, a join formula that creates a new timestamp.

Here’s my expressions formula:

join($["varDateTimeSelected_Arr[0]"],"T", regex_replace($["varDateTimeSelected_Arr[1]"], "[0-9]", "0"))

But other than expected it creates a pretty strange string with the T that separates date and time in the beginning. I don’t see a place where I have told it to do so.

Any ideas?