Hi dc,
If cell A4 contains a Date & Time value, and your formula in A5 is =A4+1, then your result in A5 will be the Date & Time value for the next day.
When a date is entered into a cell whose format is set to Automatic, Numbers will, if the date is in a format recognized as the date part of a Date & Time value, add the missing bits to make it a full Date & Time, then display only the part that has been entered, matching the style used to enter it. If text alignment in the cell has been set to Automatic, the result will be aligned to the right edge of the cell.
If Numbers does not recognize the entry as a date, it will be accepted as text, and if alignment is Automatic, will be aligned to the left edge of the cell.
Setting the cell format to Date & Time sets only the way that Date & Time values entered in that cell will be displayed. If something other than a date or a time or a full date and time value has been entered, the format setting, not able to recognize it, will revert to Automatic, and provide the most appropriate formatting.
The error message indicates that your "Date" was not recognized as a date, and was accepted as text. Here's an example showing entries of "dec 5" Sept 3 and December 1 (not yet entered):
In F2, Numbers has recognized dec 5 as a date, completed the date part with the current year, and set the time part to 00:00:00, the beginning of that day, has displayed the result according to the Data format set in the format Inspector, and aligned it right (as a numeric or number-like value).
In F3, Numbers has not recognized "Sept 3" as a date, but has accepted it as text, and aligned it left.
F4 when entered, will be treated in the same manner as F2, as it is in a format recognized by Numbers as a Date.
Try leaving the format setting for A4 as Automatic and setting the alignment to Automatic as well, then re-enering the date. If it aligns right, Numbers has recognized the format, and the formula should not produce that error message.
Regards,
Barry