RDB is a text format used by USGS for output of gage data.  These can be opened reliably in MS Windows with the WordPad program.

Text format RDB and other files can be copied and pasted into MS Excel.  If all data pastes into a single column, then use the Paste-Special
 function and choose the 'text' option.

Depending on the source, these files may vary in their arrangement of columns.
In some files, the date and time are placed into separate columns with Date in a format like 20120102 for January 2nd, 2012, and time as
 134500 for 13:45 and 00 seconds.  In this case, the values may be converted into an additional column in true Date/Time format with the
 following Excel function (assuming Date is column A, Time is column B):

=DATE(LEFT(A1,4),MID(A1,5,2),RIGHT(A1,2)) + TIME(IF(LEN(B1) > 5, LEFT(B1, 2), IF(LEN(B1)>4, LEFT(B1,1), 0)), IF(LEN(B1)=6, MID(B1, 3,2), IF(LEN(B1) = 5, MID(B1, 2, 2), IF(LEN(B1) = 4, LEFT(B1, 2), 0))), 0)