Add "when" parameter in /api/datasets to be able to retrieve datasets before or after certain date

[longshuicy]

Is your feature request related to a problem? Please describe.
Add "when" parameter in /api/datasets to be able to retrieve datasets before or after certain date. On the controller endpoint GET datasets there is this "when" parameter; it can be used in pagination to turn the page to previous or next. It is missing in the api endponit.

Describe the solution you'd like
implement the "when" parameter, when = "a", return datasets after certain date; when = "b" return datasets before certain date.

Additional context
an example of how it's currently implemented on the controller endpoint

[longshuicy]

merged in develop