What is an “open time” and how accurate is it?
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Open time is the timestamp recorded when your email's tracking pixel fires. Your ESP logs it as the moment the email was "opened." The idea is useful: knowing whether people open your email at 8am or 8pm could inform when you send. In practice, the accuracy is limited.
The pixel fires when an email client loads the image. But email clients don't always load images immediately when a person actually reads the email. Some cache images ahead of time. Apple Mail Privacy Protection pre-loads images at delivery, before the recipient ever sees the message. That means the "open time" you see for Apple Mail users reflects when Apple's servers fetched the pixel, not when a human actually read your email.
For non-Apple Mail users, it's still an approximation. Someone might open an email on mobile during their commute, then actually read it on desktop an hour later. You'd record the first pixel load as the open time, even if the meaningful engagement happened later. Gmail also pre-fetches some images, adding similar distortion.
Open time data can still be useful at a statistical level. If you're comparing two large audience segments and one consistently opens at 7am while the other opens at noon, that's a real pattern worth using for send time optimization. The noise averages out across enough data points.
But for individual open times on specific contacts, treat the data as a rough signal rather than an exact timestamp. Click events are much more reliable for understanding individual behavior, since they require an actual intentional action from the reader.
If you're building send-time optimization into your program, inbox placement timing matters more than open time data for most senders.
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