As far as I can tell, the operating system is only available to customers with select phones running Windows 8.1. It has been available on new Microsoft-made Lumia phones for awhile.

Reports of a St. Patrick’s Day release date for Windows 10 Mobile update have been circulating for awhile, but it was made official today, per a Microsoft blog.

This news comes a day after Twitter (TWTR) and Microsoft (MSFT) jointly unveiled a new Twitter app for Windows 10 Mobile.

As a follow-up, Microsoft published a list of available new phones that run the OS, all of which appear to be Microsoft Lumia models. A limited number of older Windows 8.1 phones can be updated as well.

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However, as noted on Thurrot.com, there are pretty slim pickings given that the various carriers still have to sign off on the upgrade.

Users on older phones can download an Update Advisor app to alert them if a particular phone is a candidate to run the new Windows version.

All in all, it doesn’t appear to be a very seamless or user-friendly process. That could be extra detrimental for a product that’s purportedly trying to take on market-leading Android phones or the smartphone-of-the-elite, the Apple (AAPL) iPhone.

Note: This story was updated at 3:40 p.m. EDT to reflect that new Microsoft phones running Windows 10 have been available for some time.