Power Query find minimum/maximum dates across multiple tables

Btw, looks like there's a slight confusion in the function signature:

   FnMinMaxDate = (TableName as table, ColumnName as text)

Do you want your first parameter to be table, or a table name? Looking at your parameters name and your input table, you're passing down the table's name (text), but your parameter type is table. I would recommend passing the table itself, instead of its name (check Table.Column's input parameters). So it should look like this:

   FnMinMaxDate = (InputTable as table, ColumnName as text)

And your input table:

TableName(table)    ColumnName(text)

------------------------   ------------------

Sales                          SalesDate

Sales                          OrderDate

Purchase                   PurchaseDate

Does this make sense?

And then, what you need is probably to call Table.AddColumn function (in the query editor, you would see a button "Add Custom Column" for it)

you would call it in a way like this:

= Table.AddColumn(tableWithTablesAndColumnNames, "MinAndMax", each FnMinMaxDate([Table], [ColumnName]))

One other thing, if you're returning only 1 row, consider returning just a record instead of a table with 1 row.