This basic shell script uses the raspistill command to take a shot with different settings depending on the time. It's made to be called using a crontab

fakewebcam.sh

#!/bin/bash
#fakewebcam script

TIME=$(date +"%H")
DAY='10'
NIGHT='20'
if [ ${TIME} -ge ${DAY} -a ${TIME} -le ${NIGHT} ]; then
        #echo "It's day!"
        raspistill -q 90 -w 640 -h 480 -t 60 -o /var/www/webcam.jpg
else
        #echo "It's night!"
        raspistill -q 90 -w 640 -h 480 -ex night -t 10000 -o /var/www/webcam.jpg
fi