  Strong gravitational lensing forms multiple, time delayed images of
cosmological sources, with the "focal length" of the lens serving as a
cosmological distance probe. Robust estimation of the time delay distance can
tightly constrain the Hubble constant as well as the matter density and dark
energy. Current and next generation surveys will find hundreds to thousands of
lensed systems but accurate time delay estimation from noisy, gappy lightcurves
is potentially a limiting systematic. Using a large sample of blinded
lightcurves from the Strong Lens Time Delay Challenge we develop and
demonstrate a Gaussian Process crosscorrelation technique that delivers an
average bias within 0.1% depending on the sampling, necessary for subpercent
Hubble constant determination. The fits are accurate (80% of them within 1 day)
for delays from 5-100 days and robust against cadence variations shorter than 6
days. We study the effects of survey characteristics such as cadence, season,
and campaign length, and derive requirements for time delay cosmology: in order
not to bias the cosmology determination by $0.5\sigma$, the mean time delay fit
accuracy must be better than 0.2%.
