Yes, you can change clock settings for your customer. It is supported in all plans. By default, it is set to 12 hour clock. After activating above setting, it will convert into 24 hour clock. Please checkout the following images to see the difference.
Please follow below steps to activate above setting:
Click on 'General Settings' Tab then 'Store Settings'
Under 'Calendar Widget Settings', Check '24 hr Time Format' checkbox
Check in your store to see the new changes.