Until they think warm days will never cease, For summer has o'er-brimm'd their clammy cells. (lines 10-11)
The bees have no sense of past of future: they live for the present. Does the poem argue that we should be more like the bees? Because "summer" has made it all the way inside the flowers, the time is probably around mid-day.
Or on a half-reap'd furrow sound asleep, (line 16)
No sleeping on the job, autumn! Since she has only reaped half of the furrow of poppies, we think that the day has progressed since the first stanza. This is her afternoon siesta.
Thou watchest the last oozings, hours by hours. (line 22)
From the half-way finished job of reaping, we arrive at the completely finished job of cider-making. It's almost quittin' time, and the sun will set soon.
Think not of them, thou hast thy music too,-- (line 24)
The speaker advises autumn not to look back in jealously on spring's music. Spring never seems so distant as at the peak of autumn.
While barred clouds bloom the soft-dying day, And touch the stubble-plains with rosy hue; (lines 25-26)
The progression from mid-day sunlight to sunset could be a metaphor for the progression from summer to winter. That would be using one measure of time to represent another – a neat poetic trick.