In this scene, Lady Macbeth is sleepwalking, and she seems to be, in part, reliving the night that Macbeth murdered King Duncan at her behest. On that night, the couple planned to commit the murder in the wee hours so that Duncan would be fast asleep and his grooms would be thoroughly passed out from the wine and wassail with which Lady Macbeth plied them. Just before Macbeth did the deed, he said,
The bell invites me.
Hear it not, Duncan, for it is a knell
That summons thee to heaven or to hell (2.1.75-77).
It seems that Macbeth may have heard the tolling of the clock, as his wife would have as well. Therefore, in her dreamy sleepwalking state, she seems to hear the clock chiming again, and so this is why she says, "'tis time to do it." Then, in her dream, she jumps a bit forward in time, recalling her words to Macbeth after the murder, when she says that they need to hurry up and get to bed because they would soon be called upon (and they must look like they've been asleep). At the time, she said to Macbeth,
I hear a knocking
At the south entry. Retire we to our chamber . . .
Hark, more knocking.
Get on your nightgown, lest occasion call us
And show us to be watchers (2.3.84-91).
She did not want to be caught awake, especially after she heard someone knocking. Now, in her dreams, she hears the knocking again and tells Macbeth to get into bed so that no one sees them up and about. She is reliving highlights from the night of Duncan's murder because this is one of the events that weighs so heavily on her conscience.