How to escape colon in HTML attribute with PHP rendering

I'm not sure "How to escape colon in HTML attribute with PHP rendering" is the right question, but here's the issue.

I'm implementing Events items from Schema.org and am having issues correctly rendering dates with a colon in the content attribute and it seems to be happening in the php rendering. I'm writing this in a custom Drupal blocks module.

Desired outcome is:

<p class="upcomingdate" itemprop="doorTime" content="2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>

Actual outcome is:

<p class="upcomingdate" itemprop="doorTime" content="00">Sat, Oct 15th at 8:00am</p>

In the php module file, I stripped everything dynamic out and literally hardcoded it as:

$content .= '<p class="upcomingdate" itemprop="doorTime" content="2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>';

Essentially, every individual colon I add to the content attribute only returns everything after the colon, which is why I'm thinking it needs to be escaped.

Some interesting behavior I tried was

$content .= '<p class="upcomingdate" itemprop="doorTime" content="2022-10-15T08\:00">Sat, Oct 15th at 8:00am</p>';

Which didn't work. It yielded the same actual outcome from before.

I tried this:

 $content .= '<p class="upcomingdate" itemprop="doorTime" content="2022-10-15T08\::00">Sat, Oct 15th at 8:00am</p>';

And got:

<p class="upcomingdate" itemprop="doorTime" content=":00">Sat, Oct 15th at 8:00am</p>

Which actually included the colon in the content attribute.

Then I thought, to try these with their corresponding actual outcomes:

$content .= '<p class="upcomingdate" itemprop="doorTime" content=":2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>';

<p class="upcomingdate" itemprop="doorTime" content=":2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>

$content .= '<p class="upcomingdate" itemprop="doorTime" content="test:2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>';

<p class="upcomingdate" itemprop="doorTime" content="00">Sat, Oct 15th at 8:00am</p>

$content .= '<p class="upcomingdate" itemprop="doorTime" content="::2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>';

<p class="upcomingdate" itemprop="doorTime" content="::2022-10-15T08:00">Sat, Oct 15th at 8:00am</p>

What is actually happening and how can I get that individual lone colon into my content attribute?