Automatically detect date versus datetime

You can convert all datetime columns to date as illustrated in the query below:

let
    Source = #table(type table[Date1 = datetime, Text = text, Date2 = datetime, Number = number],
        {{#datetime(2018,1,1,0,0,0),"Hello",#datetime(2018,1,2,0,0,0),1},
         {#datetime(2018,1,3,0,0,0),"World",#datetime(2018,1,4,0,0,0),2}}),
    TransformList = List.Transform(Table.ColumnsOfType(Source,{type datetime}), each {_, type date}),
    DatetimesToDates = Table.TransformColumnTypes(Source,TransformList)
in
    DatetimesToDates

Should you have 1 or a few columns that must stay on datetime, you can remove those like in the query below in which "Date2" is removed (and will stay on datetime):

let
    Source = #table(type table[Date1 = datetime, Text = text, Date2 = datetime, Number = number],
        {{#datetime(2018,1,1,0,0,0),"Hello",#datetime(2018,1,2,0,0,0),1},
         {#datetime(2018,1,3,0,0,0),"World",#datetime(2018,1,4,0,0,0),2}}),
    DatetimeColumns = Table.ColumnsOfType(Source,{type datetime}),
    FilteredColumns = List.Difference(DatetimeColumns,{"Date2"}),
    TransformList = List.Transform(FilteredColumns, each {_, type date}),
    DatetimesToDates = Table.TransformColumnTypes(Source,TransformList)
in
    DatetimesToDates