NOTE I've edited to add further logic to my question based off the first answer given.
What Seems a Correct Statement
In the LOTR movie The Fellowship of the Ring, Aragorn makes the statement upon exiting Moria that they must move quickly, because "by nightfall these hills will be swarming with orcs" (see a few seconds of the clip). This makes sense, given that orcs have issues with sunlight, and so would likely wait until night to move out of Moria.
Now it is granted that orcs can move in sunlight (based off the preceding link), but the article linked to by that link states: "They [orcs] preferred to act at night but they like all living creatures needed light to see by." I'm not sure how official that statement is, but within the movie universe, the scene of the orcs surrounding the group shows none with light on them, indicating they can move and see rather well in the dark.
One answer has ventured that the reference to nightfall had nothing to do with the fact of darkness, but rather the time frame it would take to have the hills swarming. This may be so, but given the rapid movement of orcs in Moria when they surround the group, and that one would generally assume that numerous other cracks and crevices known to the orcs near the exit of Khazad-dum (whose bridge to the exit had been severed) would not prevent their more "immediate" (rather than by nightfall) swarming of the hills, it seems questionable. So evidence that this delay till nightfall was for reasons other than sunlight needs justification.
What Seems a Questionable Statement
After leaving Lothórien, and making camp by the riverside at Parth Galen ("on the western shore of [lake] Nen Hithoel"), Aragorn says "we cross the lake at nightfall." Legolas is uneasy and says they must move on, but Aragorn says "No. Orcs patrol the eastern shore [of lake Nen Hithoel]. We must wait for cover of darkness" (watch about 50 seconds of this clip for both statements).
Given orc nature in avoiding sunlight and the apparent ability of orcs to see reasonably well in the pitch dark of a cavern (at least in the movie universe), it would seem to me that darkness is less of a "cover" to orc eyes than daylight, for in daylight the orcs are less likely to even be "outside" to see anything. This is not to mention that the group themselves would be less likely to spot the orc patrols in the dark, not to mention that the orc patrols would likely at least increase in the dark.
Is this plot element explainable in the movie universe or is it an error in the movie plot? Does the book have a similar apparent contradiction?
By my logic presented, it appears to be a lapse in Aragorn's judgment (in universe), one to which no one calls him out on. But perhaps there is some logic in universe regarding orc nature or habits that explains Aragorn's statement, making it one of good judgment.
Though the question arises from the movie, answers from the book and other Tolkien lore are acceptable and desired for support of answers.