Schweser Note says all spread strategies have limited upside and limited downside. But I think calendar spread has unlimited upside and downside.

Say for example, an investor sells a 15-day call and buys a 45-day call. If the first call is not exercised after 15 days, then the calendar spread can be seen as a single long call, which has unlimited upside. If the second call is not exercised after 45 days, the the calendar spread can be seen as a single short call, which has unlimited downside. So, unlike bull spread and bear spread, calendar spread should have unlimited upside and downside.

Is that correct?