In SharePoint calendar by default we can see the view as Day/week in 12 hour format, not in 24 hour format. We can to change the format from 12 hour to 24 hour through UI or using PowerShell commands.
We can change that through UI by following steps,
Navigate to SharePoint Site settings page and in Site Administration section click on “Regional settings” link
In Time format drop down select 24 hour format. Click on ok to save the settings.
We can change the format using following Powershell commands,
$site = Get-SPSite <SiteCollectionURL>
$web = $site.OpenWeb()