How to delete the record from CosmosDB specific days back

I have a collection activities it contains a field activityDate which contain date like 2017-02-24T19:04:18Z

I want to delete all record from this collection which are older that numberOfDays.

I am performing this task by writing cosmosdb stored procedure

I try something like

var query = 'SELECT * FROM activities a WHERE a.activityDate > = "' + numberOfDays+ '"';

I am facing problem executing the query as it's not syntactically correct

Or there can be some another way of doing it.

Thanks in advance.