Thank you Petr Degtiarev,
sorry for not being descriptive. I posted the same question again in this thread
https://discuss.elastic.co/t/date-histogram-for-bucket-aggregation/284592
funnily posted the answer as a question again. Maybe I've done something wrong when hitting the cluster. after seeing your answer only then I went back and tried my answer/question again, this time it works.
Thank you so much for your help.