Recording periods relative to - but not including - sunrise

I'd like to record for (say) thirty minutes, starting one hour after sunrise. So, in the config app I select 'sunrise' recording period, but the 'before' box doesn't accept negative values - the period must include sunrise. The only way I can see around this is to set the wrong timezone, but this will be a headache when it comes to timestamps etc. Am I missing something? Thanks!