S.D. Codified Laws § 58-18-24
Subject to § 58-18-20, every blanket health insurance policy shall contain a provision that in the case of claim for loss of time for disability, written proof of such loss must be furnished to the insurer within thirty days after the commencement of the period for which the insurer is liable, and that subsequent written proofs of the continuance of such disability must be furnished to the insurer at such intervals as the insurer may reasonably require, and that in the case of claim for any other loss, written proof of such loss must be furnished to the insurer within ninety days after the date of such loss. Failure to furnish such proof within such time shall not invalidate nor reduce any claim if it shall be shown not to have been reasonably possible to furnish such proof and that such proof was furnished as soon as was reasonably possible.
SDCL 58-18-24