"oh dark thirty" meaning in English

{
  "etymology_text": "From U.S. military terminology, in which hours before 10:00 am are prefixed with a zero, read as \"oh\", as in 0530, oh five-thirty, for 5:30 am.",
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "noun"
      },
      "expansion": "oh dark thirty",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English entries with language name categories using raw markup",
          "parents": [
            "Entries with language name categories using raw markup",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English entries with topic categories using raw markup",
          "parents": [
            "Entries with topic categories using raw markup",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English terms with non-redundant non-automated sortkeys",
          "parents": [
            "Terms with non-redundant non-automated sortkeys",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Times of day",
          "orig": "en:Times of day",
          "parents": [
            "Periodic occurrences",
            "Timekeeping",
            "Time",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "I had to get up at oh dark thirty to catch a plane.",
          "type": "example"
        }
      ],
      "glosses": [
        "Some unspecified hour in the early morning (implies an unpleasant time to be awake)."
      ],
      "id": "en-oh_dark_thirty-en-noun-Dz9QJC-8",
      "links": [
        [
          "hour",
          "hour"
        ],
        [
          "awake",
          "awake"
        ]
      ],
      "related": [
        {
          "word": "0-dark-hundred"
        },
        {
          "word": "oh dark hundred"
        },
        {
          "word": "dark thirty"
        }
      ],
      "synonyms": [
        {
          "word": "crack of dawn"
        },
        {
          "word": "stupid o'clock"
        },
        {
          "word": "wee small hours"
        },
        {
          "word": "early morning"
        },
        {
          "word": "0-dark-thirty"
        },
        {
          "word": "oh-dark-thirty"
        },
        {
          "word": "zero dark thirty"
        }
      ]
    }
  ],
  "sounds": [
    {
      "audio": "En-au-oh dark thirty.ogg",
      "mp3_url": "https://upload.wikimedia.org/wikipedia/commons/transcoded/4/4c/En-au-oh_dark_thirty.ogg/En-au-oh_dark_thirty.ogg.mp3",
      "ogg_url": "https://upload.wikimedia.org/wikipedia/commons/4/4c/En-au-oh_dark_thirty.ogg",
      "tags": [
        "Australia"
      ],
      "text": "Audio (AU)"
    }
  ],
  "word": "oh dark thirty"
}

{
  "etymology_text": "From U.S. military terminology, in which hours before 10:00 am are prefixed with a zero, read as \"oh\", as in 0530, oh five-thirty, for 5:30 am.",
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "noun"
      },
      "expansion": "oh dark thirty",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "0-dark-hundred"
    },
    {
      "word": "oh dark hundred"
    },
    {
      "word": "dark thirty"
    }
  ],
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English entries with language name categories using raw markup",
        "English entries with topic categories using raw markup",
        "English idioms",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with audio links",
        "English terms with non-redundant non-automated sortkeys",
        "English terms with usage examples",
        "en:Times of day"
      ],
      "examples": [
        {
          "text": "I had to get up at oh dark thirty to catch a plane.",
          "type": "example"
        }
      ],
      "glosses": [
        "Some unspecified hour in the early morning (implies an unpleasant time to be awake)."
      ],
      "links": [
        [
          "hour",
          "hour"
        ],
        [
          "awake",
          "awake"
        ]
      ]
    }
  ],
  "sounds": [
    {
      "audio": "En-au-oh dark thirty.ogg",
      "mp3_url": "https://upload.wikimedia.org/wikipedia/commons/transcoded/4/4c/En-au-oh_dark_thirty.ogg/En-au-oh_dark_thirty.ogg.mp3",
      "ogg_url": "https://upload.wikimedia.org/wikipedia/commons/4/4c/En-au-oh_dark_thirty.ogg",
      "tags": [
        "Australia"
      ],
      "text": "Audio (AU)"
    }
  ],
  "synonyms": [
    {
      "word": "crack of dawn"
    },
    {
      "word": "stupid o'clock"
    },
    {
      "word": "wee small hours"
    },
    {
      "word": "early morning"
    },
    {
      "word": "0-dark-thirty"
    },
    {
      "word": "oh-dark-thirty"
    },
    {
      "word": "zero dark thirty"
    }
  ],
  "word": "oh dark thirty"
}