Calendaring software is software that minimally provides users with an electronic version of a calendar. Additionally, the software may provide an appointment book, address book, and/or contact list. These tools are an extension of many of the features provided by time management software such as desk accessory packages and computer office automation systems. Calendaring is a standard feature of many PDAs, EDAs, and smartphones.
The software may be a local package designed for individual use (e.g. Lightning extension for Mozilla Thunderbird, Microsoft Outlook without Exchange Server, or Windows Calendar) or may be a networked package that allows for the sharing of information between users (e.g. Mozilla Sunbird, Windows Live Calendar, Google Calendar, or Microsoft Outlook with Exchange Server).
Calendaring software will contain one or more of the following features:
- Calendar – a calendar showing dates and days of the week. An example a simple software calendar is the Unix cal command, which simply outputs a monthly or yearly calendar.
- Address book – a list of contacts with information to enable the user to communicate with the contacts.
- Appointment attachments – This feature allow users to attached a file to an appointment. If the appointment includes other participants, the attachment is shared with them.
- Appointment calendar – a list of appointments and the attendees for the appointments. This software may include the capability of detecting scheduling conflicts, notifying the participants of the conflict, and suggesting alternate meeting times.
- Appointment reminders – Automatically reminds participants of an upcoming meeting.
- Availability sharing – this feature allows users to share their availability with others (users can select how much detail is shared); thus facilitating meeting scheduling amongst several individuals.
- Calendar publishing – some calendaring tools allow the user to publish select calendar information on a public link.
- Calendar exporting – Users are allowed to export selected calendars into various file formats, including iCalendar standard.
- Collaborative scheduling – the capability of the software to check schedules and propose meeting times to all of the participants. This allows the invitees to suggest times that will work best for them, allowing the organizer to pick a meeting time that works best for all of the participants.
- Customization – This feature allow users to customize several available features such as: email appointment remainders, calendar viewing default, workweek and work hours display, etc.
- E-mail – an electronic mail communication system. This can be tied into the appointment calendar to send reminders and notify the participants of issues arising with scheduled meetings.
- Group Calendar – a calendar showing dates of groups in addition to individual calendars.
- Multiple calendars – this feature allows users to create separate calendars (i.e. work calendar, children school calendar).
- Multi viewing – this feature allow users to select how their calendar is displayed: one day, one week, one month, or one year.
- Printing – User may print selected schedule. Usually, this feature allows users to select how she wants to have the printout to look (i.e. include comments, subject only, etc.).
- Web-based interface – allow users to access their calendars from any computer or mobile device (including cell phone) without having to solely rely on their work or personal computer.
- Chandler (PIM)
- eM Client
- eXo Platform
- Ganttic 
- Google Calendar
- IBM Lotus Notes
- Microsoft Exchange
- Microsoft Outlook
- Mozilla Sunbird
- Netscape Communicator
- Novell GroupWise
- Now Up-to-Date & Contact
- Sun Java Calendar Server
- Tom's Planner
- Windows Live Mail
- Teamwork (software)
- Yahoo! Calendar
- The Calendaring and Scheduling Consortium
- SyncML standard
- Web Calendar Access Protocol
- Personal information manager
- 65+ Online Calendars and Calendar Tools, written in Oct 2007, linked 22.09.2010