The way the Omnisci’s Crossfilter works is it takes the string that you input and it essential inserts it into an sql query. For example
var time = crossFilter.dimension("datetime") // the datetime string will be inserted directly into the SQL query
SELECT datetime FROM <database>;
Building off of this we are now able to achieve the filtering that we are after. Let’s say that we want to group by hour, then you can do
var time = crossFilter.dimension("extract(hour from datetime)")
SELECT extract(hour from datetime) FROM <database>;
Now you can use your newly grouped dimension!