Go to Database Expert and edit your command. On the right is a section titled "Parameter List". Click "New" to create your parameters BegDate and EndDate. Make sure the value type is set to Date. Then write your query like this:

SELECT blah, blah, blah FROM abc  WHERE more stuff here and c.create_timestamp >= {?BegDate}
and c.create_timestamp < ({?EndDate} + 1)

Unless there is a need to write your query manually (union or aggregate query) it is better not to use a command. Drag the required tables across and link them together in the Links tab. In your report you can then create a parameter called "Date" and make it a range parameter. In your select expert you can then write this:

{c.create_timestamp} = {?Date}

The range parameter gives you the flexibility to say if the range includes or excludes the end values, or if the range is open ended. Crystal dynamically changes the resulting query to match. Manually writen commands can't do that - if you want an open ended range and leave the enddate blank, you'll get an error. It's also easier for the user since they can enter the start and end in one place, they don't need to enter two separate parameters.

Unfortunately you pretty much have to re-write the report from scratch you can't map fields between commands and tables.