This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Not marked with or showing a date: an undated letter; an undated portrait.

adj. Being such as to have long-lasting appeal without ever going out of style: a classic, undated dress.

adj. not marked with a date

adj. of a style that will not go out of fashion; a classic

adj. Rising and falling in waves toward the margin, as a leaf; waved.

adj. Not dated; having no date; of unknown age.

Not dated; having no date: as, an undated letter or bill.

Having a waved surface; rising and falling in waves toward the margin, as a leaf; waved. Also undate.

In heraldry, same as undé.

In botany, same as undulate.

adj. not bearing a date

"It's an interesting, unpretentious, receptive personal literary experiment (pdf excerpt here), and Halliday's surprise takeaway is how remarkably, vibrantly "undated" most of the contents of Hudson Review summer '63 are."

undatable   

waved    dateless   

abated    annotated    awaited    baited    bated    belated    berated    conjugated    created    dated   

type-written    unsigned    handwritten    unframed    unencountered    typewrite    six-page    ill-written    time-lapse    unpublished