I have a formula, not sure if its the correct one for what I want to do

Column C shows the despatch date
Column H shows the method of shipment

I have the following formula in Column I

=IF(ISNUMBER(SEARCH("sea",H57)),C57+28,C57+7)&IF(I SNUMBER(SEARCH("export",H57)),C57+28,C57+7)

This should result in a new date which if the method of shipment is either
sea or export it will add 28 days to the Despatch date if not then it will
add 7 days.

Column I is formatted to date but the value I get is numeric (I think it
could be a date series)

When I just had the first criteria of "sea" it worked and the result showed
as a date but now I have added the second criteria of "export" the result is
for example 3983339833.

What am I doing wrong??
Thanks
Winnie