After posting (naturally), I realized quick 'n' dirty https timers are available from places like TimeAndDate.com. Select "secure web page" in their settings. If you already have a server where you can keep your published script, see Laura Gibbs' advice HERE. Looking forward to figuring out that method for an asynchronous class this summer -- and thank you, Laura!
For those who just want a fast timer, the HTML below is what I got from TimeAndDate. It's working just fine on a page at my Canvas site for AP World History. The downside, of course, is that clicking it brings you to an ad-supported page offering, you know, free countdown timers for websites. I haven't seen offensive ads in the banner space, but wish this didn't put students a click away from obnoxious junk like "loginhelper."
Better options for the technically challenged and hurried are appreciated.
<iframe src="https://free.timeanddate.com/countdown/i76a71ph/n263/cf100/cm0/cu4/ct0/cs1/ca0/cr0/ss0/cac000/cpc000/pcfff/tcfff/fs100/szw320/szh135/tatDays%20to%20AP%20World%20History%20Modern%20Exam/tac000/tptThe%20WHAP'M%20Exam%20happened%20on%20May%2014%2C%202020/tpc000/matSpiffy%20says%2C%20%22You%20can%20do%20this!%22/macf90/mpc000/iso2020-05-14T08:00:00/pa5" allowTransparency="true" frameborder="0" width="330" height="145"></iframe>