Dickinson is saying that the Now should be treasured and celebrated. The Now means the present. Every past and future moment was at one time, or will be, a present; a Now. Each moment in the past, was at one time, present when it occurred. Each moment in the future will be present – when it occurs. Therefore, every moment that has ever and that will ever exist in time was, is or will be a present moment; a Now. So forever or the infinite is composed of Nows because each present is a Now. That’s why she ends the poem with:
No Different Our Years would be
From Anno Domini’s—
The present Now is no different from the ‘present Now’ (it was present then) of the year Jesus Christ was born. An hour, 2,000 years ago is the same as an hour ‘now.’ Think of the timeline of all history. Each moment, when it occurred, was present: a Now. Forever is composed of these Nows.
This poem can actually be explicated further with deeper philosophical inquiry about the nature of time. I don’t know if Dickinson was writing to state the significance of all ‘presents’ (all Nows) and to appreciate life and/or the deeper understanding of time as fleeting and that all Nows will someday cease to exist, buried in the past. Or, maybe this is about the human perception of time as linear with the emphasis on the experience of life through a series of presents (Nows). This could imply that a non-human perception of time (immortal, spiritual or nonlinear) would not distinguish between past and present and would treat all moments in time as always existing, so each Now (every moment in history - from anyone's perspective) always exists, just as all spatial dimensions always exist. In this case, forever is composed of Nows and Nows are forever.
Time, death, mortality/immortality are all very common themes in Emily Dickinson’s poetry. This poem is a memento mori, reminding us of our own mortality and the fleetingness of time. She’s outright saying that forever is composed of Nows, but also that each Now is destined to become a ‘Then.’ As soon as you speak ‘Now,’ it is gone. This is the fleeting nature of human time and presence, so it is devastating but also liberating because it makes you aware of the necessity to appreciate each Now while it is ‘here.’
Everyday situation example: Yesterday, I could do nothing right. Today was the complete opposite. Yesterday was filled with one disastrous Now after another. Today was a blessing: 86,400 wonderful Nows (seconds).