dateToSerial should round output

[borgar]

Something like 18:22:35 will yield 0.7656828703693463 which has "useless" decimals beyond the ninth:

0.765682864 => "18:22:34.999"
0.765682865 => "18:22:35.000"
0.765682876 => "18:22:35.000"
0.765682877 => "18:22:35.001"

It may be misleading to emit this amount of precision when the library does not really support it in any other date handling.