tl;dr
হে রাজেন্দ্র, তব হাতে কাল অন্তহীন (he raajendra, taba haathe kaal antahiin), poem 39 from Tagore's 1901 collection নৈবেদ্য (naibedya)
Deets
"Endless Time" appears as Poem 82 in Rabindranath Tagore's Gitanjali (Song Offerings). This collection of 103 poems was originally published in 1912 by the India Society, London. Tagore's identically-titled Bengali collection গীতাঞ্জলি (giitaañjali), which literally means "Song Offerings", had appeared in 1910. However, the English collection is not a complete translation of the Bengali volume. It includes only 59 poems from the latter. The remaining 44 of the 103 poems in the English collection comprise translations of poems from several of Tagore's previously published Bengali works. All translations were Tagore's own.
"Endless Time" is a rendition of a poem that appeared in the 1901 collection নৈবেদ্য (naibedya, or Oblation). The poems in naibedya lack titles, being merely numbered. This is typical for Tagore, whose poems are usually referred to by their first lines. The source for "Endless Time" is poem 39, হে রাজেন্দ্র, তব হাতে কাল অন্তহীন (he raajendra, taba haathe kaal antahiin; my translation: "O king of the gods, in your hands is time without end".)
In 2011, William Radice published a groundbreaking edition of the English Gitanjali that included not only Tagore's renditions, but also Radice's new translations of Tagore's Bengali originals. Radice translates he raajendra as follows:
O Great Lord, Time in your hands is unending.
Beyond all counting, days and nights arrive
And pass, ages forming and then expending.
You do not permit delay, yet never strive
For speed. You know how to wait. For hundreds of years
You slowly prepare the blooming of one bud.
Time is not in our hands; our jostling careers
Grasp at what, once missed, can’t be made good.
Our service to you, Lord, only comes after
Tasks we spend too long to complete. Empty
Of gifts, alas, now falls our plate for your pooja.
But when at wrong moments, fearful and hasty,
We rush to you, we find we are not too late:
There’s time still left for us; it doesn’t abate. (p. 194)
In contrast to Tagore's free verse rendition, Radice casts the poem as a Shakespearean sonnet. Tagore's Bengali original also has 14 lines, but is in rhymed couplets, not in sonnet form per se. Nevertheless, Radice's translation actually is closer to the literal meaning of the Bengali original than Tagore's own. For example, Tagore writes:
Thou knowest how to wait.
Thy centuries follow each other perfecting a small wild flower.
The Bengali lines are:
প্রতীক্ষা করিতে জান। শতবর্ষ ধ'রে
একটি পুষ্পের কলি ফুটাবার তরে
চলে তব ধীর আয়োজন।
pratiikShaa karite jaana. shatabarSha dha're
ekaTi puShper kali phuTaabaar tare
chale taba dhiira aayojan
A literal line-by-line translation would go something like this:
You know how to wait. For a century
toward the blossoming of a single flower-bud
moves your deliberate plan.
Radice's translation follows the Bengali admirably closely:
You know how to wait. For hundreds of years
You slowly prepare the blooming of one bud.
Radice's appendices also provide a list of the sources of each poem in Tagore's English Gitanjali, and it specifies naibedya 39 as the original of "Endless Time". For anybody interested in knowing the antecedents and textual history of each poem in the English Gitanjali, as well as for those wishing for an English rendition of Tagore's Bengali originals that's more faithful than Tagore's own rather free translations, Radice's edition is indispensable.
Disclaimer
My Bengali is dreadful. It's only about my fourth language. I understand it well enough to get by, and can more or less hold my own in a conversation; but I read it only with some difficulty, and can't write it at all, though I can type it. Reading non-literary prose is difficult enough, and reading poetry ups the ante a fair bit. I've probably made deathless howlers in my attempted translations above. Rely on them strictly AYOR. Corrections are of course welcomed.
Note
Transliteration of Bengali above follows the iTrans scheme.