Here's a fun one:

Watch what happens to the result when the day goes from December 29th to December 30th.

I also tried adding -1 second with a different result.

----8<-------------------------------------------------------
Dim date, time As String
'---Load Console and DT Modules
Uses "Console"
Uses "dt"

'--- These are the strings
date="12-29-2016"
time="15:38:44"
PrintL DT_DateTimeSubSeconds(date,time,1) '--- subtract a second

'--- Advance the date
date="12-30-2016"
PrintL DT_DateTimeSubSeconds(date,time,1) '--- whoopsie!
PrintL DT_DateTimeAddSeconds(date,time,-1) '--- add (-1) second -- oof!

WaitKey
------------------------------------------------------->8----