You can edit date and time formats just like number formats. When you enter a date into a number cell, it is formatted according to the date style defined in the Base style, but you can apply a different format to it by applying a different style to the cell.
Within the Number section of the Style dialogue box, click on the right arrow at the edge of the date format definition box. The following menu will appear, containing a large selection of pre-defined date and time formats.
Pick one of these formats by clicking on it. It will be entered in the date and time definition box. If it matches your requirements, that is all you need do; however, just like number formats, you can edit date and time formats to your precise requirements as long as you obey the general rules outlined above.
Saving a date format as part of a style
You can save date formats in exactly the same way as number formats. Define a number format within a style, or create a new style specially for the time or date format you want to apply to certain cells in your worksheet.