Dictionaries mostly list
time stamp as two words, some of them with a hyphen.
The only dictionary I saw with the single word variant
timestamp as its principal entry was Wiktionary. Merriam-Webster recognizes
timestamp as an alternate form for the noun
time stamp, but has
time-stamp, with a hyphen, as the verb, with no alternative.
Nevertheless
timestamp is widely used for both the noun and the verb, so it's clear that we should allow
timestamp,
timestamping and
timestamped as valid words.
According to the OED
time stamp has been in use since 1855 as a date stamped on a letter or document, but of course it's become much more widely used in recent years for a piece of digital data accompanying a file or electronic transmission. A lot of news stories using the word are about timestamps cited as evidence of when a photo, video or sound recording was made, or the creation date of a document.
There are some signs the word is starting to creep into less digital contexts: the
Times of London wrote in December 2022 about footballer Beth Mead, recovering from an ACL injury.
“I’m two weeks post-op now and I’m in a really good position,” she said. “It’s difficult to put a timestamp on it with this injury...”
I'm going to follow my usual practice with new words, of inserting it as rare, especially in view of the prevalence of two-word versions.