Checkbox is checked even though checked=false

I want to uncheck my Checkbox with Javascript, but it didn't work. And then I tried to set the checked property to false directly in HTML, but not even that worked: (I tried checked="false" too...)

My HTML:

<input type="checkbox" id="dateCheckbox" checked=false >
<label for="dateCheckbox"> Keinen Zeitraum angeben</label>

Result:

Why is it still checked?