Death's dateless night

Question

What do you think Shakespeare meant by this expression, which occurs in one of his sonnets?

“When to the sessions of sweet silent thought
I summon up remembrance of things past,
I sigh the lack of many a thing I sought,
And with old woes new wail my dear time’s waste:
Then can I drown an eye, unused to flow,
For precious friends hid in death’s dateless night,
And weep afresh love’s long since cancelled woe,
And moan the expense of many a vanished sight:
Then can I grieve at grievances foregone,
And heavily from woe to woe tell o’er
The sad account of fore-bemoaned moan,
Which I new pay as if not paid before.
But if the while I think on thee, dear friend,
All losses are restor’d and sorrows end.”

Answer 1

Date in Shakespeare always refers to a fixed duration, and is almost always used in a context when the end of the period (usually fixed by death, literal or metaphorical) is alluded to:

And back to Athens shall the lovers wend,
With league whose date till death shall never end. — MND, III, 2

Where you may abide till your date expire. — Per, III, 4

Is not my teeming date drunk up with time? — RII, V, 2

Be brief, lest that be process of thy kindness
Last longer telling than thy kindness' date. — RIII, IV, 4

Shall bitterly begin his fearful date
With this night's revels and expire the term
Of a despised life closed in my breast
By some vile forfeit of untimely death.

Thy end is truth's and beauty's doom and date.—Son 14

Summer’s lease has all too short a date. — Son 22

Dateless, likewise, always refers to a period without an end; it means, in effect, eternal, unending

The dateless limit of thy dear exile —RII, I,3

Arms, take your last embrace! and, lips, O you
The doors of breath, seal with a righteous kiss
A dateless bargain to engrossing death! — R&J, V, 3

Cupid laid by his brand, and fell asleep:
A maid of Dian's this advantage found,
And his love-kindling fire did quickly steep
In a cold valley-fountain of that ground;
Which borrow'd from this holy fire of Love
A dateless lively heat, still to endure, — Son 153
*(Note that still here has the sense forever.)

“Death’s dateless night”, then, means “death’s endless night”--the unending night which is death.

Answer 2

He's alluding both to the fact that time has ceased for his deceased friends, and to the fact that their passing still feels as fresh to him as if it had just occurred.