d = "25" : m = "10" : y = "2019"
oCell = oSheet.GetCellByPosition(0, 0) : oCell.[How to store m & "/" & d & "/" & y in this cell with date format?]
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)