A few days ago, I quietly added the new duration parameter to Agical links, and updated the original blog post as if it had always been there.☺
Since only a lucky few people use Agical — though it's totally gratis and doesn't even require a signup/API key — this gives me an opportunity to remind you that it exists!
Using duration together with a start time (dtstart) is good when an appointment is stored in a {{lead.token}} and you don't want to have to create another custom DateTime field to hold the end time.
(In contrast, when you're using Program-level {{my.tokens}} to store event info, it's not a big deal to create 2 separate tokens for the dtstart and dtend params.)
duration is just a number followed by the letter H for hours or M for minutes. Like so (sorry, you're gonna have to scroll to grab it all):
https://ics.agical.io/?dtstart={{Lead.Next Appointment Date}}America/Chicago&duration=30M&subject=Meeting with {{Lead.Lead Owner First Name}}&description=Talk about product options&organizer={{Lead.Lead Owner Email Address}
Another thing to notice in this example is I'm telling Agical that the lead-level field {{Lead.Next Appointment Time}} is in the Marketo pod timezone — which is US Central time for all US instances, regardless of your instance timezone. And yes, that means you're probably sending out the wrong times with or without Agical if you're using DateTime fields. That's something else to dedicate a post to. Someday soon.