One major advantage to using an online calendar is the accessibility you have to the calendar at all times. Assuming you have a phone with internet access that is, you can access your calendar anytime anywhere.
This is important because if something happens and you are notified that you need to reschedule a day or event etc and you are notified of this when you are out with your family or exercising when you do not have access to the calendar.
Sometimes you have to wait until you are done with everything and leave it to your calendar holder, or an old paper calendar, or the one currently connected to your laptop. You can also promote your event online via https://thebiocalendar.com/.
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Where a lot of things happen in between before you have time to put it on your calendar. There is a high chance that you will forget to put it in your calendar and end up forgetting the changes and causing a lot of trouble.
For example, losing a big business or spending important time with family friends will cause stress, frustration, failure, and so on. Information on their calendar is not good when you need to focus on things for the day ahead.
Another thing online calendars do is eliminate confusion, especially if you have a group online calendar: for example, a shared online family calendar.