Agree with LaurentCouartou . Timeline slicer requires Date type field.
In your scenario, you can select the three columns (January 2016, February 2016 and March 2016), select Unpivot columns in Query Editor, then replace these values(January 2016, February 2016, March 2016) with appropriate Date type values as shown in the second screenshot. This way, you are able to drag the Date field(Attribute column in my example) into Timeline slicer.
Thanks, Lydia Zhang
Community Support Team _ Lydia Zhang If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.