specifies the format of dates for the columns that show dates (return the Date values)
| date | Date | the date which needs formatting | 
| column | string | the name of the column that called the template | 
gantt.templates.grid_date_format = function(date, column){
    return gantt.date.date_to_str(gantt.config.date_grid)(date);
};
The template function is called for all tasks, except for the unscheduled ones.
The grid_date_format template is used only by the date_grid template, so if you make some changes in it, grid_date_format will be affected.