Margin is a function of risk. The risk of being assigned on the short call is covered by ownership of the longer term option. You could exercise the long call in response to any assignment on the short option. The maximum risk in this spread, as shown below, is defined by the initial net debit. Imbedded in every option premium is a volatility assumption.
Volatility is the statistical measurement of a stock's price fluctuation over time.
In other words, option market participants believe XYZ will be somewhere between 75.75 and 176.75 during that time. In other words, you inherently expect option volatility to end up higher than presently imbedded levels at the call's expiration date. Meantime, selling the 17-day call implies you think this option's volatility forecast is too high, or 'rich'. A purchase of a longer term option sharing the same strike price as a written shorter option is known as a long time spread. What you're really banking on is a flat market of just such a duration as to allow for the wasting of the short option's life.