Barbara, the date of August 17, 1863, is simply the date on which the muster roll was made out.
The muster roll itself covers the activities of the regiment between January 1 through April 30, 1863, with the latter date being the "as of" date -- as of April 30, 1863, Pvt. E. C. Malone was absent sick.
Sometimes, a muster roll prepared months after the reporting period will include an update, such as, "since returned to duty," or "since died," but not many regiments did this. In most cases you can consider the "as of" date to be a snapshot of the regiment on, for example, April 30, 1863, regardless of when the roll was actually prepared.