Date
DateEpoch
type
Which date system a workbook counts its serials in: the date1904 flag of <workbookPr>.
Carried as the epoch year rather than as a boolean because it is what every conversion here actually needs, and a boolean would mean a ? 1904 : 1900 at each of them: five chances to write the ternary backwards, on a value whose wrongness is silent by construction (a date read under the wrong system is still a perfectly good date, four years off).
ts
type DateEpoch = 1900 | 1904;