Yes, we can, it will be a "custom format". The xls file format has several data formats that are standard and the rest are all written in the file as custom. If you go into Excel, the date formats look like:
*Wednesday, March 14, 01
March 14, 2001
3/14/01 1:30 PM
All the other formats are custom as:
Calc XLS has the facility to read, parse and display these custom formats from imported files. Calc XLS doesn't yet have the ability to set/write those custom formats. We should add a "Custom" format in a future release.