How to theme date combo input side-by-side?
Looking at the CSS provided by the module, the boxes are foreseen as block elements. Even the used classes on the form fields are date-no-float, so for some reason they don't want that the fields are shown in column layout ...

The fact that you used a sub theme for your admin, you can add these CSS styles (based on Seven theme) in your css file. These will create the desired result you want.

.date-no-float { float: left; width: auto; clear: none; }
.date-no-float &gt; .form-item &gt; label { float: left; margin: 34px 40px 0 30px; }
.start-date-wrapper { clear: left; }