Still, Google noted yesterday that "video ads built in Flash will not be impacted at this time" on its ad networks.
The new deadline is Google's latest move to wean the web off Flash, which has been the source of many a security scare, with an endless stream of bugs affecting the Flash Player browser plugin that has been required to view much of the web's animated and video content.
Just yesterday Adobe patched 22 critical flaws affecting the Flash Player plugin on all browsers and all desktop operating systems.
Adobe said it was not yet aware of any attacks on the flaws. However, attacks that exploit the bugs are likely to show up in exploit kits in coming weeks and months.
On the advertising front, Google has since 2014 offered free tools to convert Flash ads to HTML5 and last year began automatically converting Flash ads to HTML5, a move designed to help advertisers reach smartphones that don't support Flash.
Also, to improve Chrome's performance, Google updated the browser last September to automatically pause Flash-animated ads while allowing Flash video to continue running, drawing a line between 'central' and 'non central' content.
While Google hasn't moved entirely off Flash for advertising, the deadline is another milestone on its demise on the web.
Both YouTube and Facebook have now moved entirely to HTML5 for video on the respective platforms, and Adobe helped Netflix prepare its recently released HTML5 player for Firefox, bringing its support in line with the existing HTML5 player for IE, Edge, Safari, and Chrome.
Even Adobe is moving away from Flash, yesterday releasing Animate CC, its more HTML5-friendly output tool replace to Flash Professional.