Death's dateless night

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.”

Solution 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.