when convert many time strings to unixtimestamp, i get 10% percent records converted wrong

[xiuxiuxiaodi]

I have a table of 2 millions reconds with column name time1, the record is like"2017-10-30 11:11:11", when i use "select time1, unix_timestamp(time1) as time1unix from xx", I found i get much the datetime string row records converted right, but there are some wrong converted.
At first , I think the unix_timestamp function is not thread-safe, then I write a user-defined function with thread-safe, but I get some wrong converted also, thougth not the same wrong order with unix_timestamp function.
I find this error , because my record is sorted, then i use the unix_timestamp(time1) records , it's not.
Can anyone tell me why?