  The existing CMS endcap calorimeters will be replaced with a High Granularity
Calorimeter (HGCAL) for operation at the High Luminosity (HL) LHC. Radiation
hardness and excellent physics performance will be achieved by utilising
silicon pad sensors and SiPM-on-scintillator tiles with high longitudinal and
transverse segmentation. One of the major challenges of the HL-LHC will be the
high pileup environment, with interaction vertices spread not only in position,
but also in time. In order to efficiently reject particles originating from
pileup, precision timing information of the order of 30 ps will be of great
benefit. In order to meet such performance goals, the HGCAL will provide timing
measurements for individual hits with signals above 12 fC (equivalent to 3-10
MIPs), such that clusters resulting from particles with $p_T$ > 5 GeV should
have a timing resolution better than 30 ps. Given the complexity and size of
the system, this poses a particular challenge to the readout electronics as
well as to the calibration and reconstruction procedures. We present the
challenges for the front-end electronics design, results from prototype tests
in laboratory and beam environments, as well as anticipated timing performance
from simulation.
