Passing AWK variables to shell commands for evaluation

For some reason, I never seem to get how to use the shell from within GNU AWK to manipulate the AWK variable and return it. Spending a lot of time with the system and getline command, I still couldn't figure out how to pass a variable to a shell command, evaluate it and then return the value.

After much trial and error and googling, this seems to work.
a=20090601
cat ../data/C_100119.dat | \
awk -v compdate="$a" '{
while ("date --date="$4 " +%Y%m%d"|getline aa)
{
if(aa >= compdate)
print $0
else
exit
}
}'

where I'm extracting records whose 4th column date value is greater than a.

Hope this is useful to others.

Update: Scratch the above code. I got some feedback about the correct ways to use getline.
a=20090601
awk -v compdate="$a" '
{
if (("date --date="$4 " +%Y%m%d"|getline aa) >0){
if(aa >= compdate) {
print
next
}
}
exit
}' ../data/C_100119.dat



is a better way to do it.