The ActiveHome Pro timer function allows timers to be applied to specific days of the week (e.g., Monday and Thursday). It lacks the ability to apply timers to what is referred to as "skip days." One application where skip days is necessary is with landscape sprinklers.
Common sprinkler timers available on the market today such as those from Irritrol, Toro and Rainbird allow a sprinkler valve to be set to come on either on specific days of the week or to come on every X number of days (i.e., skip days). The skip day option means that a user can, for example, set a sprinkler valve to come on every 3rd day without regard to what day of the week it is.
Where lawn sprinkler valves are normally set to come on certain days of the week due to regular mowing schedules (e.g., mow every Saturday so do not want the water to come on Saturday morning), in contrast plant and tree sprinkler valves are normally set to come on every X number of days, again without regard to days of the week. For example, a shrub valve may be set to come on every 4th day and a tree valve to come on every 10th day. The idea is that skip days allow for a pure regular watering schedule (i.e., the same number of days pass between each watering).
Further, a skip day option should allow for the skip day number to be set by the user up to say 30 (e.g., activate valve every 30 days; maybe virtually no limit is possible given just a loop). Hence, the user could set the skip day option from 1 to any number up to 30.
On a side note, with typical sprinkler timers the number set is not actually a skip day but rather means that the valve should come on every X-day. So, if set to 1 the valve comes on every day and not every other day (means that it comes on every 1st day). If set to 3 it means that the valve comes on every 3rd day (skipping only 2 days). However, probably fine if the number actually denoted skip days and in this case entering a 0 would mean that the valve would come on every day (i.e., 0 skip days between on days) and entering 2 would mean the valve would come on every 3rd day (i.e., 2 skip days between on days).
On an interface note, within the ActiveHome Pro interface the user should be able to see what day each valve is currently on in its cycle. Also, again with common timers, the valve actually comes on the first day of the cycle and not the X-day (e.g., if a valve is set to come on every 7th day, it actually comes on when the timer reaches 1 again and not when it reaches 7). But, effectively doesn't really make a difference which method is used.
User's guides for common sprinkler timers could also be referenced when designing a skip day option for ActiveHome Pro.