How to add X days to a date in ETEXT template

Hello,

I am creating an ETEXT template where I have no control of the data definition so have to work with what I get in XML output.

Following is part of the template where I have two date fields - creation date and expiration date:

The source of creation date is InstructionInfo/InstructionCreationDate and this works fine but for exparation date I would like to add x days to InstructionInfo/InstructionCreationDate. Is this achievable in ETEXT template? N.B. I cant change the source data.

You can see in the screenshot that I have tried using xpath function date-time-add-days but this did not work (not that it produced any errors but the output was not what I expected).