You could also divide now() by the number of seconds of the day (86400) to get a sort of absolute day since ever (1/1/1970)
(now()/8640)%2==0 //gives true if today is even
(now()/8640)%3==0 // true every third day
In my case, I allocated an int counter for this, set up an alarm to every day 6am and increment the counter at the alarm. Every time the counter hits %x==0 I would reset the counter and perform the actions I need every x days. For an every other day case, I used a simple bool type. This way I can easily see how many days are left to my event occur and alter it by external interfaces, e.g. telnet server.
Just finished my first arduino project and filled a UNO flash memory! This is great guys, great forum and great platform!
In this one I used the ethernet shield to get the time via Network Time Protocol (ntp), set up alarms to water my herbs (nothing illegal hehe) every other day, set up alarms to post temperature and soil moisture via http POST requests, a telnet server responds to basic commands such as get temperature, get moisture, set watering interval. I have a 12v water pump that is turned on via relays during the watering interval. Hope to blog the project soon!