System.schedule() method can be used to schedule a Schedulable class at specified time instantly in Salesforce.
Check the following sample code for reference.
Sample Code:
AccountUpdateScheduler obj = new AccountUpdateScheduler();
String cron = '0 30 15 29 10 ? 2015';
System.schedule('Testing', cron, obj);The above code will execute AccountUpdateScheduler exactly at 3:30 PM on 20th October 2015.
To know more about scheduling, please check the following link