Exporting a Sunbird calendar into Thunderbird+Lightning

I spent ages writing my 2007 paper diary into Sunbird. Then I discovered Lightning as a Thunderbird extension. I would like to suck all the painful data straight out of SB into TB+L. However depending on the application neither of the suggestive "Import" or "Export" menu items seems (to me) to work in the way that one might have expected or hoped. Am I doomed to stick with separate SB and TB, or duplicate the awful labour of building the calendar in TB+L or can anybody instruct me how to achieve the desired endpoint quickly and efficiently? Thank you.
PS I'll post this only on the TB support forum, as I guess duplicating it in SB would be annoying.