Replacing a block of text in a file with sed

I need to locate and replace a block of text in a file daily.

for example,

....locate following block

 start_year                      =  2004
 start_month                   =     12
 start_day                       =     31
 start_hour                      =    00
 end_year                        =  2005
 end_month                     =     01
 end_day                         =     02
 end_hour                        =     12

.... and replace it with this:

 start_year                      =  2005
 start_month                   =     01
 start_day                       =     01
 start_hour                      =    00
 end_year                       =  2005
 end_month                    =     01
 end_day                        =     03
 end_hour                       =     12

Surely I don't have to do a line-by-line replacement. It seems like I should be able to pick a beginning and ending element, and choose to replace everything in between... I don't know how to deal with a block in sed, any ideas....?

Thanks in advance