date-fns the reason npm build fails on Next.js? - reactjs

Question

This is the offending line of code:
<p>
{(isSameDay(event.start, event.end)) ? `On ${format(event.start, 'LLL do')}`: `From ${format(event.start, 'LLL do')} to ${format(event.end, 'LLL do')}`}
</p>
I also tried:
<p>
{(isSameDay(new Date(event.start), new Date(event.end))) ? `On ${format(new Date(event.start), 'LLL do')}`: `From ${format(new Date(event.start), 'LLL do')} to ${format(new Date(event.end), 'LLL do')}`}
</p>
and:
<p>
{(event.start === event.end) ? `On ${format(new Date(event.start), 'LLL do')}`: `From ${format(new Date(event.start), 'LLL do')} to ${format(new Date(event.end), 'LLL do')}`}
</p>
The last one comes back false when it should be true, so it doesn't even matter that it fails the build.
I also uninstalled and re-installed date-fns.
I'm able to use isBefore, isAfter, isToday on a different page, so I have no idea why this line in particular is messing me up. Is my logic wrong? What am I missing? I love date-fns and would prefer not to use a different library, and I definitely don't want to deal with dates with vanilla JavaScript, but if I can't figure this out that's probably what I'll end up doing.
Here is the output when I try to build:
info - Using webpack 5. Reason: Enabled by default https://nextjs.org/docs/messages/webpack5
info - Skipping validation of types
info - Creating an optimized production build
info - Compiled successfully
info - Collecting page data
[== ] info - Generating static pages (20/175)undefined start
undefined end
Error occurred prerendering page "/vendor/events/EventsCard". Read more: https://nextjs.org/docs/messages/prerender-error
TypeError: Cannot read property 'start' of undefined
at EventsCard (D:\boho\front\app\.next\server\chunks\9136.js:57:28)
at d (D:\boho\front\app\node_modules\react-dom\cjs\react-dom-server.node.production.min.js:33:498)
at bb (D:\boho\front\app\node_modules\react-dom\cjs\react-dom-server.node.production.min.js:36:16)
at a.b.render (D:\boho\front\app\node_modules\react-dom\cjs\react-dom-server.node.production.min.js:42:43)
at a.b.read (D:\boho\front\app\node_modules\react-dom\cjs\react-dom-server.node.production.min.js:41:83)
at Object.exports.renderToString (D:\boho\front\app\node_modules\react-dom\cjs\react-dom-server.node.production.min.js:52:138)
at renderPage (D:\boho\front\app\node_modules\next\dist\server\render.js:596:45)
at Object.ctx.renderPage (D:\boho\front\app\.next\server\pages\_document.js:53:30)
at Function.getInitialProps (D:\boho\front\app\.next\server\chunks\6859.js:603:19)
at Function.getInitialProps (D:\boho\front\app\.next\server\pages\_document.js:60:56)
info - Generating static pages (175/175)
> Build error occurred
Error: Export encountered errors on following paths:
/vendor/events/EventsCard
at D:\boho\front\app\node_modules\next\dist\export\index.js:487:19
at runMicrotasks (<anonymous>)
at processTicksAndRejections (internal/process/task_queues.js:95:5)
at async Span.traceAsyncFn (D:\boho\front\app\node_modules\next\dist\telemetry\trace\trace.js:60:20)
at async D:\boho\front\app\node_modules\next\dist\build\index.js:833:17
at async Span.traceAsyncFn (D:\boho\front\app\node_modules\next\dist\telemetry\trace\trace.js:60:20)
at async D:\boho\front\app\node_modules\next\dist\build\index.js:707:13
at async Span.traceAsyncFn (D:\boho\front\app\node_modules\next\dist\telemetry\trace\trace.js:60:20)
at async Object.build [as default] (D:\boho\front\app\node_modules\next\dist\build\index.js:77:25)