Call a Function within ListView, within Child Page

I have a Function to take the date value supplied by the Twitter API (eg Fri Jan 24 06:16:51 +0000 2014) and convert it to a different format using datetime.tostring (the output will be simiar to '6.16am Friday 24th Jan', and the purpose of the function is to correctly parse the date and then apply the correct suffix to the date.
I'm calling this function directly from the .aspx page, passing in an Xpath variable from an XML file.  However, I'm consistantly recieving this error:  'TwitterDateConvert' is not declared. It may be inaccessible due to its protection level.
The function is being called from within a ListView, which is on a child page to a Masterpage.  I'm suspecting that this hierachey is causing my issue somewhere.
The aspx code:
<div class="item-attribution-datetime"><%# TwitterDateConvert(XPath("created_at"))%></div> ...