An Atomic Clock with 10−18 Instability. N. Hinkley, J. A. Sherman, N. B. Phillips, M. Schioppo, N. D. Lemke, K. Beloy, M. Pizzocaro, C. W. Oates, A. D. Ludlow. Science. Vol. 341 No. 6151 (13 September 2013): 1215-1218.
strontium
Jun Ye Group. JILA — a joint institute of University of Colorado Boulder and the National Institute of Standards and Technology (NIST).
A quantum network of clocks. P. Kómár, E.M. Kessler, M. Bishof, L. Jiang, A.S. Sørensen, J. Ye, M.D. Lukin. Nature Physics. Vol. 10 (August 2014) 582–587.
Rotation of the Earth. Nigel Bunce & Jim Hunt. The Science Corner. No. 42 (13 September 1984)
Coral Growth and Geochronometry. John W. Wells. Nature Vol. 197 No. 4871 (9 March 1963): 948–950. Growth rings in fossil coral can be used to determine the length of the day in the past (by counting the number of solar days in a year).