timestamps only work when written in double quotation marks but not in single or as plain number

[MaggiWuerze]

when i get an array via ajax the calendar can't or won't recognize it.
here is my example data:

[
  {
    "start": "1551610800174",
    "end": "1551625200174",
    "title": "example1",
    "content": "Demo Bemerkung",
    "category": "Aussendiensttermin"
  },
  {
    "start": "1551697200174",
    "end": "1551711600174",
    "title": "example2",
    "content": "Demo Bemerkung",
    "category": "Aussendiensttermin"
  },
  {
    "start": "1551524400174",
    "end": "1551538800174",
    "title": "example3",
    "content": "Demo Bemerkung",
    "category": "Aussendiensttermin"
  }
]

if i add events like this it works:

calendar.addEvents(
 [
  { // An event on the current week on Wednesday from 10h to 12h
   start: "1551265200",
   end: "1551279600",
   title: 'An event title !',
   content: 'Hello World!' 
   category: 'A test category name'
  }
 ]);