mktime and tm_isdst

I saw a lot of different views so thought of asking here.

I read man mktime:

 (A positive or zero value for tm_isdst causes mktime() to presume initially
 that summer time (for example, Daylight Saving Time) is or is not in
 effect for the specified time, respectively.  A negative value for
 tm_isdst causes the mktime() function to attempt to divine whether summer
 time is in effect for the specified time. 

My question is, shouldn't tm_isdst be kept as -1 to let the system decide if its dst or not and that way the code becomes dst agnostic?

Am I missing something?