Absolute dates can be expressed as follows:
by the Middle to Late Devonian (ca 385 Ma), fish had ...
between 55 ka and 21 ka
formed by 3,500 Ma
older than 4,000 Ga
where ka = kiloannum (a thousand years ago), Ma = mega-annum (a million years ago) and Ga = giga-annum (a billion years ago).
Note that, in expressing a range of time, the largest (oldest) number occurs first.
Duration of time can also expressed in kiloyears (kyr), megayears (Myr) and gigayears (Gyr):
at rates of 20–30 m per Myr
for the past 300 Myr
The Jurassic Period lasted approximately 50 Myr [duration] from ca 201 Ma to ca 152 Ma [absolute dates].
The before present (BP) timescale is often used in geology; see Date and time.
