"a. d." meaning in Latin

{
  "forms": [
    {
      "form": "a.d.",
      "tags": [
        "alternative"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "la",
        "2": "phrase",
        "head": "a. d."
      },
      "expansion": "a. d.",
      "name": "head"
    }
  ],
  "lang": "Latin",
  "lang_code": "la",
  "pos": "phrase",
  "senses": [
    {
      "alt_of": [
        {
          "word": "ante diem"
        }
      ],
      "categories": [
        {
          "kind": "other",
          "name": "Latin entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "bold_text_offsets": [
            [
              0,
              5
            ]
          ],
          "english": "3rd February",
          "text": "a. d. III[.] Nōn. Febr.",
          "type": "example"
        }
      ],
      "glosses": [
        "Initialism of ante diem."
      ],
      "id": "en-a._d.-la-phrase-6J6X0ZaD",
      "links": [
        [
          "ante",
          "ante#Latin"
        ],
        [
          "diem",
          "diem#Latin"
        ]
      ],
      "tags": [
        "abbreviation",
        "alt-of",
        "initialism"
      ]
    }
  ],
  "word": "a. d."
}

{
  "forms": [
    {
      "form": "a.d.",
      "tags": [
        "alternative"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "la",
        "2": "phrase",
        "head": "a. d."
      },
      "expansion": "a. d.",
      "name": "head"
    }
  ],
  "lang": "Latin",
  "lang_code": "la",
  "pos": "phrase",
  "senses": [
    {
      "alt_of": [
        {
          "word": "ante diem"
        }
      ],
      "categories": [
        "Latin entries with incorrect language header",
        "Latin initialisms",
        "Latin lemmas",
        "Latin multiword terms",
        "Latin phrases",
        "Latin terms spelled with .",
        "Latin terms with usage examples",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "examples": [
        {
          "bold_text_offsets": [
            [
              0,
              5
            ]
          ],
          "english": "3rd February",
          "text": "a. d. III[.] Nōn. Febr.",
          "type": "example"
        }
      ],
      "glosses": [
        "Initialism of ante diem."
      ],
      "links": [
        [
          "ante",
          "ante#Latin"
        ],
        [
          "diem",
          "diem#Latin"
        ]
      ],
      "tags": [
        "abbreviation",
        "alt-of",
        "initialism"
      ]
    }
  ],
  "word": "a. d."
}