How to deduct one day from the date in xpath?

I write automated tests using SOAPUI free version. I have such format of date which I take from sql database:

2015-11-30 13:38:58.387

I need to convert it to a ormat like:

2015-11-30T13:38:58.387 or 2015-11-30

And deduct one day from it, so that to get the date minus one day.

I tried to use previous-day()function and substring-before() (using XPath):

previous-day(substring-before((//*:Results/*:ResultSet/*:Row/*:VALIDFROM/text()), ' '))

But it always gives an error like:

net.sf.saxon.trans.XPathException:XPath syntax error at char 87 {../*:Row/*:VALIDFROM/text()..}: Unknown system function previous-day()

Why doen't these functions work?

Thank you