Unfortunately, there is no way to get around this. There is a fixed limit in PB that the maximum duration of an updating trend is 7 days. If you take a look at an 8 day trend you can see that the line on the right is solid and not dotted (which means it's not updating with new values). A somewhat unintentional side-effect of this is that relative times are only returned for updating trends. If the trend has an absolute time range (which for an 8 day trend ends up being true), the absolute times are returned.
Thanks for the fast reply David, i will look how to deal with this