 Digital thermostats may also have a user-programmable air filter change reminder; this counts the accumulated run-time of the heating/cooling system and reminds the user when it is time to change the filter.The feature often displays the accumulated run-time either as an aggregate of both heating and cooling or displaying each time separately.Some digital thermostats have the capability of being programmed using a touch-tone telephone or over the Internet, such as the Nest Learning Thermostat.