Lightning synchronizes a calendar with any CalDav provider. I understand that icloud has a caldav URL, But apple do a very good job of hiding it. /blackberry say the server names is one of 10

caldav.icloud.com or p0-caldav.icloud.com to p09-caldav.icloud.com

This guy has a script that he says will extract the relevant details for you. THe good part of that is your get the card dav details as a bonus and using the sogo connector can synchronize the contacts as well.

So far I have been yapping apple like a fan boy, I am not. Gmail and Yahoo as well as GMX all offer free and reasonably well documented caldav implementations. A google search will probably list dozens of caldav providers are varying cost. and with varying support arrangements. Free often has less than ideal support due to the lack of money no sale price entails. A bit like Thunderbird really.

Personally I use Google and GMX. Simple to setup and reliable.

Now you will need to export your existing calendar to ICS format that import it to Gmail. Then create a gmail calendar in your iphone and Thunderbird. It is really the only way to merge the old an new. Now you can have the same calendar in your laptop, desktop ipad and phone.

Thunderbird also has addone that will synchronise contact with Gmail so you can keep those in sync as well.

I have left out Microsoft's Hotmail/outlook.com offering as Microsoft do not offer CalDav, only their proprietary protocols.

I took so long to reply to this because I was somewhat overwhelmed.

I'd like to take the approach you did, but need a little more guidance on 1. How to you export the existing calendar to ICS format? 2. How would you then import it to gmail? 3. How do you create the gmail acct in t-bird. . . I thought it supported lightning? 4. How about setting up the gmail calendar on the ipad as well. HOw do you do that? How does this sync? Do I need that addon you reference, or is there just a single gmail calendar in teh 'cloud?' Sorry to be so dense and I appreciate your help.

Well, I played around and got some of this resolved. I exported the file to an ICS file and imported it into a gmail calendar and it's now there.

I set up another calendar in Tbird and pasted in the URL google told me to use into "Location" filed in the Tbird calendar properties. The problem is that I can't get it to show any of the data that's been imported into the gmail calendar.

If I try to add an event into the ICS calendar (it's the only one showing) it adds it to the regular calendar and then shows all the appointments from it.

If I go into the calendar view (events and tasks/calendar) and check just the new calendar there are no events there. Also, on the left where it shows all my calendars there's a yellow exclamation point next to the name of the gmail calendar.

okay, now I've installed the provider for Google Calendar add-on and the calendar seems to sync between Tbird and gmail calendar (I haven't done anything on the iPad yet.) That's the good news. The bad news is that my tasks don't appear. I don't know if they didn't get exported or something else is going on. Does gmail calemdar even support tasks?

Tasks are not calendars. see https://addons.mozilla.org/en-us/thunderbird/addon/google-tasks-sync/