I think possible your platform have a bug...   one out of the lines below does not compile: 

1. string2 ="2018-03-11 02:17:17";
   
2. info string2;
   
3. info string2.toTime(); //this line does not compile; very very strange
   

1. string3 ="2018-03-12 04:15:17";
   
2. info string3 ;
   
3. info string3.toTime();