A day alert pops up whenever you have a task that is missing a duration estimate. Clicking the alert lets you see all the tasks and projects missing an estimate so you can assign one. Unlike "Missing Day Estimate Alert" this snippet checks for all your tasks, not just ones scheduled for the selected date.

Once there's a task without a duration estimate, the following alert will show up at the top of your main view.

Here's how it works in action. Once you click the "Assign now" button you'll see tasks without duration estimates. Then you can add time to them and when there are no more tasks without duration estimates the alert button will disappear.
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This workflow snippet uses the following strategies:

The smart list used in this workflow snippet selects all tasks without duration estimate that aren't on the backburner.

If you wish to change the color of the "Assign now" button, or the text that appears there, you can do so by going to ☰ -> Strategies, finding "Smart List Day Alerts" strategy and clicking on the cog.

If you want to change the color, you can do so by hovering the blue circle with your cursor and then clicking on it. That way you can also add an icon to the button.