The one used in print is the long dash («—»), but almost everyone on the Internet uses the hyphen («-») because other types aren’t easily available on the keyboard. The middle dash shouldn’t be marked wrong either. By the way, in this sentence the dash isn’t necessary at all unless you want to make an emphasis on something («мой» or «в конце июля»).
I didnt even know there are three different types of horizontal dashes. I was assuming the hifen sign as the correct character for these cases and Word-like processors would change it to a long one just for making it clear there is a hifen there that is not used in word binding like i did with "Word-like". Well, that may make me go back to my own language's grammar book for checking hehehe (Portuguese)