shadow_lastmod.sh

#!/bin/sh

# Have you ever wondered how to calculate the third column listed
# in the shadow password file?  It is the date of the last change
# to that user's password.  The field is calculated by looking at
# the number of days that have passed since January 1, 1970.  The
# expiration fields all depend on a good value for that field.

# January 1, 1970 is 18000 in epoch time format in seconds
THAT_WAS_THEN=18000

# Now, figure out today in seconds
THIS_IS_NOW=`perl -e 'print time()'`

# Find the difference to see how many seconds have gone by
DELTA=`expr $THIS_IS_NOW - $THAT_WAS_THEN`

# There are 86400 seconds in a day.  Convert what we have into days.
# Keep in mind that expr will truncate the decimal value down to the
# nearest integer (unlike "bc -l" for instance).  However, this is
# what we want and the same behavior the shadow password file follows.
DAYS=`expr $DELTA / 86400`

# Show it it to the monkey on the other end of the keyboard
if [ $# -gt 0 ]; then
    echo "Sample /etc/shadow entry:"
    echo "  $LOGNAME:<encrypted_password>:$DAYS:0:90:14:::"
    echo "$DAYS days have passed since January 1, 1970."
else
    echo $DAYS
fi