Creating a new mongoose model date value by adding two existing separate values within the model

Question

I'm currently working on an ExpressJS project that involves mongoose for structuring data, but in particular, I want to use dates as a big part of this project. What I'd like to have in the end is a new mongoose model item created by adding two existing values within the schema together upon creation. So, if a user creates this item (Time), the desired field (end) should be automatically created similar to a createdAt field. So if a user creates a Time item for 3 hours at 9 am, the end value should automatically be created as 12 pm.

Below is an example of what I want to accomplish:

const timeSchema = new mongoose.Schema({
  // Start Date/Time filled in a form
  date: {type: Date},
  // Length of period
  hours: {type: Number},
  // I want to create an end value by adding these two together
  end: {type: Date,
    default: mongoose.Schema.date + mongoose.Schema.hours * 60 * 60 * 1000,
    ref: "Time",
  }
})

module.exports = mongoose.model("Time", timeSchema);

I've attempted a few different methods of combining the elements, but sadly, had no success. All help is much appreciated.

TLDR: Is it possible to create a new date value by adding two existing schema items together?