deposit_contract_factory.sol

pragma solidity ^0.6.0;

import "../deposit_contract.sol";

contract DepositContractFactory {
  bool public deployed;
  uint public deploymentTime;
  DepositContract depositContract;

  constructor(uint _deploymentTime) public {
    deploymentTime = _deploymentTime;
  }

  function deploy() external {
    require(block.timestamp >= deploymentTime, "not yet!");
    require(!deployed, "already deployed");
    depositContract = new DepositContract();
    deployed = true;
  }
}

Export depositContract and use immutable:

pragma solidity ^0.6.0;

import "../deposit_contract.sol";

contract DepositContractFactory {
  bool public deployed;
  uint public immutable deploymentTime;
  DepositContract public depositContract;

  constructor(uint _deploymentTime) public {
    deploymentTime = _deploymentTime;
  }

  function deploy() external {
    require(block.timestamp >= deploymentTime, "not yet!");
    require(!deployed, "already deployed");
    depositContract = new DepositContract();
    deployed = true;
  }
}

And this one adds an extra check on the resulting codehash (if we are not to trust the factory enough):

pragma solidity ^0.6.0;

import "../deposit_contract.sol";

contract DepositContractFactory {
  bool public deployed;
  uint public immutable deploymentTime;
  bytes32 public immutable depositContractHash;
  DepositContract public depositContract;

  constructor(uint _deploymentTime, bytes32 _depositContractHash) public {
    deploymentTime = _deploymentTime;
    depositContractHash = _depositContractHash;
  }

  function deploy() external {
    require(block.timestamp >= deploymentTime, "not yet!");
    require(!deployed, "already deployed");
    depositContract = new DepositContract();
    // Soon to be supported as `address(depositContract).codehash`
    bytes32 codehash;
    assembly {
       codehash := extcodehash(depositContract)
    }
    require(codehash == depositContractHash, "broken!?");
    deployed = true;
  }
}

Additionally could use CREATE2:

depositContract = new DepositContract{salt = <salt>}();

Export depositContract and use immutable:

👍

Additionally could use CREATE2:

yeah, but the address is already predictable regardless

I reckon actually CREATE2 would be a bad idea if the salt exists in the factory because the contract could be deployed outside the factory.