How to find the rows with missing date range

Anonymous I am not aware of a way to do this completely in DAX. Some of it will need to be done in Power Query. Would that be ok?

Here's what I have done so far:

adjusted your source data for my locale

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("XY7BDYAwDAN3ybuSnYTSdJaq+68BFRAEz/Pdw2OIShE1sMJoXLCB+wWz3L6CLX0D4+cjpxMc7K+PFVBBfwJnwifIB9YT5jwA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Store code" = _t, #"Bill Start Date" = _t, #"Bill End date" = _t]),
    #"Added Custom" = Table.AddColumn(Source, "Start Date", each Date.FromText([Bill Start Date],"en-gb")),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "End Date", each Date.FromText([Bill End date],"en-gb")),
    #"Changed Type" = Table.TransformColumnTypes(#"Added Custom1",{{"Start Date", type date}, {"End Date", type date}})
in
    #"Changed Type"

Added two helper columns identifying the earliest start date and latest end date per store.

MinDate = CALCULATE(min('Table'[Start Date]),ALLEXCEPT('Table','Table'[Store code]))
MaxDate = CALCULATE(max('Table'[End Date]),ALLEXCEPT('Table','Table'[Store code]))

Created calendars for each interval

Gaps = 
var s = [Store code]
var a = SELECTCOLUMNS(Filter(ALL('Table'),'Table'[Store code]=s),"Interval",COUNTROWS(CALENDAR('Table'[Start Date],'Table'[End Date])))
return CONCATENATEX(a,[Interval],"|")

(COUNTROWS and Concatenatex is for testing)

Next step would be to UNION all the calendars and then "subtract"  them from the Min/Max calendar via EXCEPT. That would leave you with all dates that are not covered by bills.

Then the next step would be to group these gaps into a more user friendly output.

But - I don't know how to UNION all table values in a table column. The required DAX function does not exist (as far as I know).