Basic Points

• A Good till Canceled (GTC) order is an order type which stays open until closed. 
• Depending on the broker, a GTC order can stay in effect outside of the regular trading hours (9:30am-4pm EST). Some brokers may require an additional “Ext.” flag for executing outside normal hours, with the idea being that GTC orders would simply stay open from one day to the next, but only work during the day. 
• Generally, a GTC order would be canceled after 6 months–as a safeguard against staying open forever and therefore avoid creating surprises and unwanted orders. 
• In contrast, a Day order will only last until the end of the cash session. 

Advanced: Risk Considerations

If using a GTC order on a spread or complex options position, and you leg out of it by closing one of its parts, then make sure to cancel the GTC orders on the combo, or else there might be surprises later on from stranded orders on combos which were broken by legging out of the combo.

In general, there is an aggregating risk if placing many GTC orders over time because, if you lose track of how many you had or get many fills at once, then this can add up to far more total exposure than you initially expected, such as being surprised at the checkout of a grocery store when the total bill from so many bids adds up. In the case of the stock market, many bids all over from several months ago could come back and haunt you in a major crash by filling them all at once and giving you too much size. This can easily happen since GTC orders are “a type of limit order that is active until it is filled or canceled. As opposed to a day order, a GTC order can remain active for an indefinite number of trading sessions” (Cottle, 2006, p. 381). Please keep this in mind to help losing any money from unforced errors. 

Expert: Algorithmic Use

Good-to-Close can also be a parameter for more complex order types which use it as a parameter type called time-in-force. Such a parameter communicates the basics of a GTC order which is how a GRT order will remain “active until it can either be executed [filled] or is canceled by the customer” (Natenberg, 2015, p. 545). For example, a limit or stop-limit order could specify that it is GTC, and then it will continue to be a standing order day after day. 

If instead, a Day order is used (algorithmically or on limit orders outside of current market prices), then that order would get ignored if touching that level outside regular trading hours.