A required act may be unrestricted by time or confined to a specified period. Even among time-bound acts, the function of time is not uniform.
For prayer, time can act as a container (zarf), within which other acts may also be performed. The day of Ramadan fasting can act as an exclusive measure (mi'yar) dedicated to that fast.
This difference affects the treatment of having an intention at all versus identifying a particular obligation. Cases involving sickness, travel, performance, make-up fasting and specified vows must not be flattened into one rule.
A correct analysis identifies the relevant period, its legal function and the person's legal circumstances within it.