Can the Eaton PT18M-W-K (1800-Watt 15 Amp) timer be used for a 1500-Watt baseboard heater? According to the specs, the answer appears to be "yes". However, "heater" is conspicuously absent from the list of devices that can be connected to it.
The device is rated at 1800w or 1/4 HP motor loads so I would say it should be fine. I like to have a higher rating if possible but this is rated for halogen lighting. It's basically the same as a resistive load like the nichrome wire used as an element in most electric heaters.