Измените h (: mm) tt и / h: mm {- h: mm} на H в fullcalendar.js
setDefaults({
allDaySlot: true,
allDayText: 'all-day',
firstHour: 6,
slotMinutes: 30,
defaultEventMinutes: 120,
axisFormat: 'H', //,'h(:mm)tt',
timeFormat: {
agenda: 'H' //h:mm{ - h:mm}'
},
dragOpacity: {
agenda: .5
},
minTime: 0,
maxTime: 24
});