如何找到年龄大于30岁的员工人数？由于没有年龄字段，我们必须从出生日期开始

Question

如何查找年龄大于30岁的员工人数？由于没有年龄字段，因此必须从出生日期（DOB）开始查找

需要找到年龄大于30岁的员工人数，以下是mongoDB集合。

{ 
    "_id" : NumberInt(3), 
    "bloodGroup" : "B+", 
    "createdAt" : ISODate("2018-05-05T15:39:43.735+0000"), 
    "createdBy" : NumberInt(1), 
    "dob" : ISODate("1976-06-14T18:00:00.000+0000"), 
    "emergencyContactNumber" : NumberInt(1720361402), 
    "emergencyContactPerson" : "Natalie", 
    "emp_id" : NumberInt(3), 
    "fatherName" : "John Wick", 
    "gender" : "Male", 
    "homePhone" : NumberInt(1720361402), 
    "isCompleted" : true, 
    "isDeleted" : false, 
    "maritialStatus" : "Married", 
    "motherName" : "Victoria", 
    "nationality" : "British", 
    "personalEmail" : "abc101003@yopmail.com", 
    "personalMobileNumber" : NumberInt(1777770358), 
    "religion" : "Christian", 
    "updatedAt" : ISODate("2018-10-03T05:25:00.557+0000"), 
    "updatedBy" : NumberInt(3)
}