When comparing Rainlendar vs SmartDay, the Slant community recommends SmartDay for most people. In the question“What are the best calendar apps for Mac OS X?” SmartDay is ranked 7th while Rainlendar is ranked 11th. The most important reason people chose SmartDay is:
The app doubles as a task manager and allows dragging and dropping tasks into the calendar on specific days. If a task does not get completed on that day, it automatically gets rescheduled.
Specs
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Pros
Pro Integration
Your calendar data doesn't need to be stored with your computer. Data can be retrieved from Google, RTM, and even Outlook. (With the pro version removing annoying unregistered marks)
Pro Blends in with the desktop
This calendar stays out of your way, while remaining readily available to tell you what's coming up.
The Shadow skin make it look like a wallpaper calendar, but has the benefit of being interactive.
Pro Customizable
Take control of your calendar by scripting new functionality, take skins already created, or just disable all the fluff.
Rainlendar provides a platform to present your data in the way you desire.
Pro Integrates task-management with the calendar
The app doubles as a task manager and allows dragging and dropping tasks into the calendar on specific days. If a task does not get completed on that day, it automatically gets rescheduled.
Pro Aims to provide a complete organizational solution
The app includes a way to manage tasks, take notes, set reminders and manage project from within the calendar.
Pro Calendars can be shared
Cons
Con Can be unstable
The calendar crashes from time to time when entering new data.
Con it's not free
not as pricey as others, but still not free.
Con Doesn't integrate
Does not support Google Calendar integration.
Con Support is limited
The app support is confusing and hard to navigate.
Con Lacks natural language input
Natural language input allows creating events in the same way if you had to tell it to a friend. Writing something like "Lunch at 2pm" would add an event called "Lunch" at 2 PM. SmartDay lacks that functionality.