  This paper investigates postmortem timestamp reconstruction in environmental
monitoring networks. In the absence of a time-synchronization protocol, these
networks use multiple pairs of (local, global) timestamps to retroactively
estimate the motes' clock drift and offset and thus reconstruct the measurement
time series. We present Sundial, a novel offline algorithm for reconstructing
global timestamps that is robust to unreliable global clock sources. Sundial
reconstructs timestamps by correlating annual solar patterns with measurements
provided by the motes' inexpensive light sensors. The surprising ability to
accurately estimate the length of day using light intensity measurements
enables Sundial to be robust to arbitrary mote clock restarts. Experimental
results, based on multiple environmental network deployments spanning a period
of over 2.5 years, show that Sundial achieves accuracy as high as 10 parts per
million (ppm), using solar radiation readings recorded at 20 minute intervals.
