Taking uniqure value from three different columns

 =UNIQUE({K5:K80;L5:L80;M5:M80})

sort in ascending order the transponsed unique dates

 =TRANSPOSE(sort(UNIQUE($L$2:$L$5001),1,true))

Importrange to import large data

 =QUERY({IMPORTRANGE("1ZTtsquwfJIlTJisS7gD_13u4Evfcz4Nr-T9N9p3-SYQ/edit#gid=847332267","Action Data!$A$23000:$H$54002");

IMPORTRANGE("1ZTtsquwfJIlTJisS7gD_13u4Evfcz4Nr-T9N9p3-SYQ/edit#gid=847332267","Action Data!$A$54003:$H$78005")}, 

"select Col2,Col4,Col5,Col8 where Col1='"&B32&"' and Col3= '"&B11&"' and Col2 >= Date '"&TEXT(A2,"yyyy-MM-dd")&"' and Col2 <= Date '"&TEXT(B2,"yyyy-MM-dd")&"'")

How to use Google query with "order by"

 =QUERY(IMPORTRANGE("1ZTtsquwfJIlTJisS7gD_13u4Evfcz4Nr-T9N9p3-SYQ/edit#gid=620793866","Call Data!$B$20000:$K$40000"),"select Col4,Col5,Col10 where Col1= '"&B40&"'and Col4 >= Date '"&TEXT(A2,"yyyy-MM-dd")&"' and Col4 <= Date '"&TEXT(B2,"yyyy-MM-dd")&"' order by Col4",0)