You can assign a "cell format" to a cell in your spreadsheet to control how the data stored in that cell is displayed.
AppSheet uses cell formats to deduce the data type of each column in your spreadsheet. By applying the same format to all non-header cells in each column, you make it easier for AppSheet to determine the column's type.
In this example, the Google Sheets Format menu assigns a cell format to all cells in the Date column.
Note: Make sure that your column headers are bolded.