Build 3.5.0.57 of xlsgen improves how custom dates are inserted. Before this build, if you ran a statement such as :

worksheet.Date[row][col] = "15 august 2014";

xlsgen would actually parse the date by using the user's regional settings (Windows / Control panel / Regional settings), which means a date such as the one above would parse correctly ONLY if the user was using a US or UK regional settings. Otherwise this would return an error.

There was no way to specify a date format to help the parser.

Beginning with this build, xlsgen takes into account the existing style in the target cell when it parses a date to be inserted in that cell.

So if you pass a date format such as,

IXlsStyle s = worksheet.NewStyle();
s.NumberFormat = "dd-MMMM-yyyy";
s.Apply();

You can then use this :

worksheet.Date[row][col] = "15-august-2014";

and this date will be parsed correctly.