An act of worship may become binding without its command fixing a particular time for fulfilling it. Such worship is called untimed worship1. “Untimed” describes the performance time; it does not mean that the obligation is optional.
For example, someone makes this vow without naming a day:
لِلَّهِ عَلَيَّ صَوْمٌ
“I owe Allah a fast.”
The vow makes a fast binding, but the wording does not select a day for it. What must be done and which day it must be done are separate questions.
1. Untimed worship (Ibadat Ghayr Muwaqqatah): worship whose command does not prescribe a particular performance time.
- An Obligation Without a Set Performance Time