How to get the number of tweets containing a specific hashtag on a specific period of time?

I'd like to do that in javascript and draw some curves from it

you can enter this kind of search query on Twitter:
"#surfing since:2018-03-29 until:2018-04-01"

then select "latest", this link can be copied and used with to collect the data you need.

good luck! :)

Thanks! I'll try it :)