Does time I spent in the U.S. as an F-1 student count toward the H-1B substantial presence test later?

Days you spent as an F-1 exempt individual don't count toward the substantial presence test during your exemption years, but that exemption is capped at five calendar years. If you've already used up those five years as a student, you were likely a resident alien even before switching to H-1B, and your days simply keep counting. If you switch to H-1B before exhausting the five years, your exempt student days still don't count, but your new H-1B days count immediately, which can tip you into resident status partway through the year. Tracking exactly which years you claimed the exemption is essential. IRS Publication 519 details the interaction at irs.gov.