BASIC+Calc : How to convert string to date

edit

Edited to account for the M/D/Y format mentioned in the question

dim oCell as object, oLocale as object, nFmtId as Long
oCell = ThisComponent.CurrentController.ActiveSheet.GetCellByPosition(0, 0)
oLocale = ThisComponent.NumberFormats.getByKey(oCell.NumberFormat).Locale
nFmtId = ThisComponent.NumberFormats.queryKey("MM/DD/YYYY", oLocale, False)
if nFmtId = -1 then _
   nFmtId = ThisComponent.NumberFormats.addNew("MM/DD/YYYY", oLocale)
oCell.NumberFormat = nFmtId
oCell.Value = DateSerial(2019, 10, 25)