Why does my crontab job fail? 0 0 * * * some command > /var/log/mylog.`date +%Y%m%d`
In many versions of crontab, the percent sign (%) is treated specially, and therefore must be escaped with backslashes:
0 0 * * * some_user some_command >"/var/log/mylog.$(date '+\%Y\%m\%d')"
See your system's manual (crontab(5) or crontab(1)) for details. Note: on systems which split the crontab manual into two parts, you may have to type man 5 crontab or man -s 5 crontab to read the part you need.