If I want to put on a calendar spread as a limit order at interactive brokers, am I allowed to specify a limit price for the spread, or do I need to specify limit prices for each of the individual legs?
I don't know about IB, but in general, and in my experience with Schwab, that's exactly how you'd enter such orders.
Bid/Ask spreads can be pretty crazy, you often see a strike showing Bid $2/ Ask $3 or a similar B/A. If the other leg of the trade is $1/$2, you might get filled as high as $2 or as low as $1. Best to enter a limit price to 'split the difference' on the spreads. If it doesn't fill as you hope you can inch up the price.