Gollum entering moria

[Kalimac]

The "night eyes" of the Orcs certainly were good, but their "day eyes" weren't necessarily so. The only Orcs we know of who were at all comfortable in daylight were the Uruk-Hai, and there's no reason to suppose that the Orcs of Moria were related. So Gollum could have slipped in during the daytime, or maybe early morning/early evening when it was still fairly light out but the Yellow Face wasn't as painful to him. Of course Gollum hated being out in the light as well, but as we see in TTT, he could stand it if he had to. Desperate times call for desperate measures and all that, and trying to sneak past an Orc guard was certainly desperate enough. I'd guess he did it in the daytime, when they were fewer and less alert.