Inverse of mat2Rdate. Real matchbox capture history tables
store t0/tEnd as MATLAB datenums (days since year 0), which
is what chtToSNRinfo()'s default timeCol='t0' and
cde()'s timeCol=NULL auto-detection both assume for any
column literally named t0. Simulated data is naturally POSIXct, so
simsTocaptureHistoryTable uses this to produce a genuinely
matchbox-compatible t0/tEnd, rather than a same-named column
that looks right but isn't the format those defaults expect.