According to this fantastic tutorial about Cron in openshift I know how use it.
Depends on the accuracy you want to achieve.
In example, in .openshift/cron/minutely/certain_date.sh
#!/bin/bash
DATE=`date +%D` #MM/DD/YY
HOUR=`date +%H`
MINUTE=`date +%M`
if [ "$DATE" == "12/21/14" -a "$HOUR" == "23" -a "$MINUTE" == "15" ];then
(
#date >> $CUSTOM_REPO_DIR/certain_date.txt
cd $CUSTOM_REPO_DIR ; #/usr/local/zend/bin/php
$CUSTOM_PHP cron_certain_date.php
)
fi