Different ways are available to achieve this -
You may have mapping table -
* inline [
Use Dual(Applymap('Days_Map',actualfieldname),Actualfieldname) as Newfieldname
Dual will help to use the same field in calculation as well, in visualization it will show the days in text.
If you have date field available and you have possibility to create days field from that then you can use date functions Day(Date) function to have the same dual field directly.