how to clear out std and transfer stderr to std?

I have the following command in a bash script,

time=$((./a.out) 2>&1)

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When run it, there are some printout in std when running a.out alone. How to clear those printout and make $time variable only contain execution time?

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time=$(./a.out 3>&1 1>/dev/null 2>&3)

This:

1. redirects stderr to an unused stream (3),

2. silences stdout

3. redirects the temporary stream (3) to stdout

This is based on this answer.