I did find the following command:

"OrderDuration"
-GTC order duration in seconds.

This would appear to be the same as GTD.

However doesnt appear to cancel the GTC - with IB at least.

Code snippet:
Lots = 100;
OrderDuration = 30;
setf(TradeMode,TR_GTC);
OrderLimit = roundto(0.999*priceClose(),PIP);
ThisTrade = enterLong();