Microsoft has been working to migrate its Outlook.com users to a new Outlook.com mail client since May 2015. And the process is proving more complicated than the Softies expected.

Last we heard, Microsoft's self-imposed goal for completing the migration of the vast majority of Outlook.com users to the new experience was end of August 2016. I asked on August 15 if this was still the goal, and a spokesperson told me "Microsoft still expects the vast majority of accounts to be upgraded by the end of summer."

But I guess something's happened in the past few days, as it now is looking like Microsoft isn't going to meet that goal -- at least for Outlook.com customers who have shared calendars.

Outlook.com user @gwydionjhr (Joel R.), who is still on the old Outlook.com, received a message today that indicated the Oulook.com upgrade will now be finished "in the first half of 2017."

The full message he received on August 28 from Microsoft:

"We're currently upgrading Outlook.com accounts. During the upgrade, you won't be able to share your calendar with some accounts, but you can still send a link to your calendar by clicking 'Get a link.' We expect the upgrade to be finished in the first half of 2017. We apologize for the inconvenience and thank you for using Outlook.com."

Microsoft announced plans to make Outlook.com more like its "regular" Outlook mail client in May 2015. At that time, Microsoft officials said they'd start making a preview of the new Outlook.com available to a small group of customers, and then broadening the preview "in the coming weeks."

By making Outlook.com more like Outlook, Microsoft execs said they'd bring a host of new capabilities to Outlook.com, including support for "Clutter" for de-cluttering inboxes; mail themes; automatic link preview generation; pop-out read and compose windows; support for add-ins like Uber, PayPal, and Maps, and more. Suggested contacts and automatic flight notifications would become part of the Outlook.com feature set, too.

In April 2016, officials admitted their initial upgrade schedule was over-ambitious. As of the end of April 2016, Microsoft had migrated only 175 million Outlook.com accounts out of a total of 400 million existing Outlook.com accounts to the new experience.

The existing Outlook.com service was running on a legacy infrastructure. The new Outlook.com is using "the same building blocks as the rest of Office 365," The result will be that new features can more quickly and easily be introduced across both Outlook/Office 365 and Outlook.com, moving forward, rather than building twice for two different platforms.

I asked Microsoft officials again today, August 28, for an update on the Outlook.com migration timetable. No word back yet. I will update this post if and when I hear back.

Update (August 29): A Microsoft spokesperson said the date in the pop-up reported above was incorrect. But Microsoft officials are not willing to say what the correct date is or even how much after the end of August it is.

Here's the official statement, from that spokesperson:

"The error message incorrectly stated the date as the first half of 2017; we're updating this message to reflect the latest migration schedule. We are in the final stages of the Outlook.com migration, with nearly 90% of our active users already migrated. A small percentage of our active users will take additional time to migrate based on certain features they are using like shared calendars."

So we still don't know what "additional time" means, at this point....