From Xojo Documentation
| This item was deprecated in version 2019r2.
Please use DateTime.SecondsFrom1970 as a replacement.
The number of seconds since 12:00AM, January 1, 1904, local time.
TotalSeconds is the "master" property from which other ways of expressing date/time are derived. A negative value of TotalSeconds indicates a Date/Time prior to January 1, 1904. Very, very large values of TotalSeconds will cause the value to be set to zero.
On Windows (due to the Windows API being used), it is not possible to go back further than 1 January 1601.
You can use TotalSeconds to calculate elapsed seconds: