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?