Fixing Surveys page and date time helper for calculating delta days

This should fix that issue on the survey page and in the helper. I think we just need to fetch DateTimeZone objects for these rather than try to make setTimezone cast them (as it might have previously worked in PHP of yore). However, I am lightyears away from a localization dynamo, so @mlocati would you mind double checking that this is sane? I'm not sure if 'GMT' is deprecated or whatnot. Thanks!). Re: #760