I know ADS-C has a timestamp, but I think you're right that ADS-B doesn't.
ADS-B does have a timestamp:
Column Information
A UTC Time Stamp
B FR24 internal unique flight ID
C Callsign KGL9268 = flight 7K9268
D Latitude
E Longitude
F Internal FR24 data type
H Squawk – Code ATC is using to identify flight
I Ground Speed in Knots
J Heading
K Vertical Speed in feet per minute
L Internal FR24 data identifier
M The ID of the FR24 receiver picking up data. (7K9268 was covered by multiple receivers)
N Internal FR24 data identifier
Source:
Crash of Metrojet Flight 7K9268 | Flightradar24 Blog