请让我知道这是否适合你。

input.txt  
1999-01-01 12:08:56  
1999-01-02 12:08:57  
1999-01-03 12:08:58  
1999-01-04 12:08:59  

PigScript:  
A = LOAD 'input.txt' using PigStorage(' ') as(date:chararray,time:chararray);  
B = FOREACH A GENERATE CONCAT(date,'T',time) as myDateString;  
C = FOREACH B GENERATE ToDate(myDateString);  
dump C;  

Output:  
(1999-01-01T12:08:56.000+05:30)  
(1999-01-02T12:08:57.000+05:30)  
(1999-01-03T12:08:58.000+05:30)  
(1999-01-04T12:08:59.000+05:30)  

Now the myDateString is in date object, you can process this data using all the build in date functions.

Incase if you want to store the output as in this format 
(1999-01-01T12:08:56)  
(1999-01-02T12:08:57)  
(1999-01-03T12:08:58)  
(1999-01-04T12:08:59)

you can use REGEX_EXTRACT to parse the each data till "."  something like this  

D = FOREACH C GENERATE ToString($0) as temp;
E = FOREACH D GENERATE REGEX_EXTRACT(temp, '(.*)\\.(.*)', 1);
dump E;

Output:
(1999-01-01T12:08:56)  
(1999-01-02T12:08:57)  
(1999-01-03T12:08:58)  
(1999-01-04T12:08:59)