timeCurrent="$(date --iso-8601=seconds)" ; # Produce `date`-parsable current timestamp with resolution of 1 second.
timeNextDay="$(date -d "$timeCurrent next day" --iso-8601=date)"; # Produce timestamp of beginning of tomorrow with resolution of 1 second.
secondsUntilNextDay="$(( $(date +%s -d "$timeNextDay") - $(date +%s -d "$timeCurrent") ))" ; # Calculate seconds until closest future midnight (res. 1 second).
timeCurrent="$(date --iso-8601=seconds)" ; # Produce `date`-parsable current timestamp with resolution of 1 second.
timeNextDay="$(date -d "$timeCurrent next day" --iso-8601=date)"; # Produce timestamp of beginning of tomorrow with resolution of 1 second.
secondsUntilNextDay="$(( $(date +%s -d "$timeNextDay") - $(date +%s -d "$timeCurrent") ))" ; # Calculate seconds until closest future midnight (res. 1 second).