Calculating 30 days average based on selected date range in date slicer

ckhoo2007 , you need measure bucketing

you need to create a measure

days = datediff(max(table[date]), selecteddate(Date[Date]), day)

or

days =

var _max = maxx(allselected(Date) , Date[Date])

return datediff(max(table[date]), _max, day)

then create an independent table with min, max and bucket

0 , 30, "  0 -30 days"

add others to the table. Use enter data

You need to use this table in new measures, where you need a group level

refer my blog or view for steps

Dynamic Segmentation Bucketing Binning
https://community.powerbi.com/t5/Quick-Measures-Gallery/Dynamic-Segmentation-Bucketing-Binning/m-p/1387187#M626

Dynamic Segmentation, Bucketing or Binning: https://youtu.be/CuczXPj0N-k