A little theory I found: Kaneki's counting the day on the calendar because he's on a timer due to the influence of Eto's kakuja on him. The other two people Eto's given her kagune to are both depicted in situations involving clocks, we also know that she herself left her kakuja for Kaneki, wondering about his hand and whether he liked her gift or not. Did she know something like his monster hand would happen? What if a side effect of Eto’s kagune is a time limit on the person, forcing them to return to Eto to minimize or postpone any ill effects? Having Kaneki physically tied to her would surely ensure her to have his strength at her complete disposal.