Code.gs

function parseTime(text) {
    {
        const match = /(\d\d?)時(\d\d?)分/.exec(text);
        if (match && match.length === 3) {
            return [match[1], match[2]];
        }
    }
    {
        const match = /(\d\d?)時/.exec(text);
        if (match && match.length === 2) {
            return [match[1], 0];
        }
    }
    {
        const match = /(\d\d?):(\d\d?)/.exec(text);
        console.log(match);
        if (match && match.length >= 3) {
            return [match[1], match[2]];
        }
    }

    return [0, 0];
}

function parseDate(text) {
    const [h, m] = parseTime(text);
    {
        const match = /(\d*?)年(\d*?)月(\d*?)日/.exec(text);
        if (match && match.length >= 4) {
            return new Date(match[1], match[2] - 1, match[3], h, m);
        }
    }
    {
        const match = /(\d*?)月(\d*?)日/.exec(text);
        if (match && match.length >= 3) {
            const year = (new Date()).getFullYear(); // 年がなければ今年にする
            return new Date(year, match[1] - 1, match[2], h, m);
        }
    }
    {
        // yyyy/mm/dd
        const match = /(\d\d\d\d)\/(\d\d?)\/(\d\d?)/.exec(text);
        if (match && match.length >= 4) {
            return new Date(match[1], match[2] - 1, match[3], h, m);
        }
    }
    {
        // mm/dd
        const match = /(\d\d?)\/(\d\d?)/.exec(text);
        if (match && match.length >= 3) {
            const year = (new Date()).getFullYear(); // 年がなければ今年にする
            return new Date(year, match[1] - 1, match[2], h, m);
        }
    }
    {
        if (text.indexOf('本日') >= 0) {
            const dt = new Date();
            return new Date(dt.getFullYear(), dt.getMonth(), dt.getDate(), h, m);
        }
    }

    return null;
}

function trigger(tweet) {
  const calendar = CalendarApp.getDefaultCalendar();
  const text = tweet.text;
  const lines = text.split('\n');
  const title = lines[0];
  const startTime = parseDate(text);
  if (!startTime) {
    return;
  }
  
  const option = {
    description: text
  }
  
  calendar.createEvent(title, startTime, startTime, option);
}

function doPost(e) {
  var params = JSON.parse(e.postData.getDataAsString());
  trigger(params);
}