Bug 711289: Time Zone Handling is Inconsistent between 2.4 and 2.5

Also Bug 699977: Since Last Run Assistant Using Wrong Time Zone in
Release 2.5.1.

Fix an error in the formatting of TZ offsets to g_time_zone_new().

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23396 57a11ea4-9604-0410-9ed3-97b8803252fd