The cells containing the text-formatted strings are C1 and D1.

C1 contains: 2016:08:23 18:43:05
D1 contains: 2016:08:21 17:39:36

so I changed your formulas to:

=MIN(DATEVALUE(SUBSTITUTE(LEFT(C1:D1,10),":","/")))
=MIN(1*RIGHT(C1:D1,8))
=MIN(DATEVALUE(SUBSTITUTE(LEFT(C1:D1,10),":","/"))+1*RIGHT(C1:D1,8))

When I paste those new formulas into my spreadsheet, the three results are #VALUE!

Why don't I get usable results?