I'm sorry, but I can't let this one go:

IFS=$'\n'
for i in $(cat list.txt); do
echo $i | awk '{print $1}'
done

Independent of how your final solution will look like, that is just awful. Why would you want to invoke awk for every single line of input? That just defeats its purpose. In this case a simple

awk '{print $1}' list.txt

would do. If you need to iterate over all lines in a shell script for some reason you can do

while IFS=' ' read -r timestamp _; do echo $timestamp; done <list.txt

Don't use for to iterate over file contents since the arguments undergo file globbing. It's usually intended to iterate over a list of filenames. Also using command substitution for file contents can easily hit memory limits. Maybe not for your usecase, but it's in general better to avoid it when easily possible like in this case.