pragma solidity ^0.4.24;

interface AlarmWakeUp {
    function callback() public;
}

contract AlarmService {
    mapping(uint => address[]) private _events;

    function set(uint _time) public returns (bool) {
        _events[_time].push(msg.sender);
    }

    function call(uint _time) public {
        address[] addresses = _events[_time];

        for(uint i=0; i<addresses.length; i++) {
            AlarmWakeUp(addresses[i]).callback();
        }
    }
}

contract AlarmTrigger is AlarmWakeUp {
    AlarmService private _alarmService;

    function AlarmTrigger() {
        _alarmService = new AlarmService();
    }

    function callback() public {
        // Do something
    }

    function setAlarm() public {
        _alarmService.set(block.timestamp + 60);
    }
}