Hi @Poppy10 ,
there is a bit more to think about - is this going to be a recurring journey or not? If not, people once suppressed, will not go back on the journey even after 70 days.
However, if you just wanted a dynamic segment with a people who travelled in the last 30 days, you can do something like that (replace account with your booking or journey, etc).

On a different note, for behavioural segments you can use a sliding window as such -

But you can't do that for a demographic segment.
Hope this help. I don't really know the structure of your data to help more. Sorry.
Thanks,
-DD