Default Day View of SharePoint Calendar control has its start hour set to 7am and end hour to 5pm.
To change this you'll need to update SPRegionalSettings.WorkDayStartHour and SPRegionalSettings.WorkDayEndtHour properties. And it is not something you can do from UI.
For example, to change start hour to 6am and end hour to 6pm you'll have to write and run this code
using (SPSite site = new SPSite("http://<YourSiteUrl>"))
using (SPWeb web = site.RootWeb)
SPRegionalSettings regionalSettings = web.RegionalSettings;
regionalSettings.WorkDayStartHour = 360;
regionalSettings.WorkDayEndHour = 1080;
The possible values you can set for WorkDayStartHour and WorkDayEndHour are 60 x (hour value in 24 hour format).
Basically, they are total number of minutes. For example,
6am = 6 x 60 = 360
6pm = 18 x 60 = 1080
9pm = 21 x 60 = 1260 and so on..
As you can see, the start hour of Day View in calendar has now changed
Hope it helps