grep file foo | while read line ; do echo "$line" | date %s.%N ; done
More readably in a script:
grep file foo | while read line
do
echo "$line" | date %s.%N
done
For each line of input, read will put the value into the variable $line, and the while statement will execute the loop body between do and done. Since the value is now in a variable and not stdin, I’ve used echo to push it back into stdin, but you could just do date %s.%N "$line", assuming date works that way.
Avoid using for line in `grep file foo` which is similar, because for always breaks on spaces and this becomes a nightmare for reading lists of files:
find . -iname "*blah*.dat" | while read filename; do ....
would fail with for.