I think I'm close, I need to do a check before continue on to next step, if I use a Do while I get into a loop I can't get out of and if I use IF it continues on.

I have a start date that needs to greater than the end date so here are the ways of written:
Thanks

if startdate > enddate then
 messagebox("whatever...) 
endif

do while startdate > enddate
 messagebox("whatever...) 
 loop
 exit
enddo