If you export the web part and locate the JSON string that contains the "refinerConfigurations" you can change the "useDefaultDateIntervals":false
and then set your own intervals like "intervals":[-365,-180,-30,-7,-1]. Dates in the past are negative and the future are positive.
If you don't know how to export and import a web part let me know and I'll add that detail to the answer.
A Display Template "~sitecollection/_catalogs/masterpage/Display Templates/Filters/Filter_SliderBarGraph.html" is responsible for displaying the refinement Panel. The Display Template gets its parameters from the Web Part.
When you export the "Refinement Panel" Web Part, you will see this section:
The section "userDefaultDateIntervals":true forces the display. You will need to change that to false in order to have the option to alternative date intervals.
So, even though -180 is a default value in the refiner type, it is not a default "Interval" to be shown in the "Display Template"
When you examine the bar chart that is controlling a Date-based Refiner, you will also notice that each bar takes 2 values rather than a single one in order to display a "date range". For example one of the bar charts take from "one month" to "one year". This means the values in the web parts are not mapped one to one to the display template. but 2 values from the refiner is used to construct "one" refinement options.
The separation of parameters configure in the refiner from the parameters that govern the display helps prevent one key issue: you do not want someone to specify large number of values in the refinement search schema other than the default which ends up damaging the display. For example, you do not want to have 20 bars in the left side of the screen and very little space to show the data on a standard Monitor.