There’s a bit of a long-standing bug in Textpattern, and its handling of time zones. It’s a bit of a head-scratcher to get your head around, so bear with me. It’s probably best described with a simple example, so here goes:

• Our writer/site admin lives in the UK. It is British Summertime. His time zone is therefore GMT+1.
• Textpattern is installed on a server in Australia. Its time zone is GMT+10.
• The site admin sets the time zone in TXP to his own GMT+1 and writes a few articles.
• British Summertime ends, so the writer’s time zone is now just GMT. Daylight Savings Time (or whatever) starts in Australia, making it GMT +11.
• All the date-based permalinks in TXP go all to hell. Most appear a day out of line.
• New stuff that gets written is inaccessible as TXP can’t decide if the publish time has passed or not.

It’s a strange one, and a bit of a pain in the ass. There’s been some discussion in this forum thread, but no consensus on how to fix it or even if it is a bug. The common workaround – which isn’t an ideal one for many people – is to just keep to one time zone and ignore anything like DST, BST or other “time modifiers”.