Linux

Paste this into Bash:

get_screen_time() {
since=${1:-today}
until_opt=${2:+--until "$2"}

journalctl --output json -u systemd-logind --since "$since" $until_opt | \
jq -r 'select(.MESSAGE_ID and .__REALTIME_TIMESTAMP) |
if (.MESSAGE | test("Lid opened|Starting systemd-logind|Operation .suspend. finished")) then
  .__REALTIME_TIMESTAMP + " start"
elif (.MESSAGE | test("Lid closed|Stopping systemd-logind|system will suspend")) then
  .__REALTIME_TIMESTAMP + " stop"
else
  empty
end' | \
awk -v current_time=$(date +%s%6N) '
{
    if ($2 == "start") {
        last_start = $1
    } else if ($2 == "stop" && last_start != 0) {
        total += ($1 - last_start) / 1000000
        last_start = 0
    }
}
END {
    if (last_start != 0) {
        total += (current_time - last_start) / 1000000
    }
    seconds = int(total)
    hours = int(seconds/3600)
    minutes = int((seconds%3600)/60)
    printf "%02d:%02d", hours, minutes
}'
}
PS1='$(get_screen_time) '$PS1

Now you have screen time for today in your prompt:

00:21 user@asus:~/Documents$ 
00:21 user@asus:~/Documents$ cd ../Pictures/
00:21 user@asus:~/Pictures$

Cool?