You can use the timeFormat option to change the format of the time string displayed in the input field and the menu items in the combobox. Available modifiers are:

h
the hour without leading zero in 12-hour format.
hh
the hour with leading zero in 12-hour format.
H
the hour without leading zero in 24-hour format.
HH
the hour with leading zero in 24-hour format.
m
the minutes without leading zero.
mm
the minutes with leading zero.
s
the seconds without leading zero.
ss
the seconds with leading zero.
p
AM or PM

To get standard 24-hour format you can use { timeFormat: 'HH:mm:ss' } as the options object when initializing the plugin.