How to truncate char string fromt beginning and replace chars in string by other chars in C or C++?

Hi,

I have a datetime char string returned from ctime_r, and it is in the
format like ""Wed Jun 30 21:49:08 1993\n\0", which has 26 chars
including the last terminate char '\0', and i would like to remove the
weekday information that is "Wed" here, and I also would like to
replace the spaces char by "_" and also remove the "\n" char. I didn't
know how to truncate the string from beginning or replace some chars
in a string with another chars without using a loop through one char
by one char of the string. I used the below code to achieve
replacement of " " by "_" and also removed the last "\n" char, without
considering removing the first 4 chars i.e. weekday information yet.
But even this I still didn't get what i like. Below is the code i
wrote:

#include <time.h>
#include <stdio.h>

int main(void)
{
   time_t ltime;
   char buf[50];

   // Get the time
   time(&ltime);

   // The datetime string returned by ctime_r is in the format
   // of "Wed Jun 30 21:49:08 1993\n\0"
   printf("The time is: %s", ctime_r(&ltime, buf));

   // replace the " " and ":" in the datetime string
   // by "_"
   buf[7] = "_"; // " ", line 18
   buf[10] = "_"; // " ", line 19
   buf[13] = "_"; // ":", line 20
   buf[16] = "_"; // ":", line 21
   buf[19] = "_"; // " ", line 22
   buf[24] = "\0"; // remove the last \n char, line 23

   // printf the new datetimestring
   printf("The time is: %s", buf);

}

When I complied it with gcc, i got the below warning:

test_ctimer.c: In function `main':
test_ctimer.c:18: warning: assignment makes integer from pointer
without a cast
and same warning for line 19 to 23 too.

When I run it, I got:

The time is: Thu Aug 7 15:02:32 2008
The time is: Thu AugX 7X15X02X32X2008Z

Can anyone kindly help me? I searched on the internet and it seems C
library doesn't have a function to truncate from the beginning? and it
also doesn't have a function for characher replacement. Should I have
to use a loop?

Thanks a lot for the help in advance.

Hongyu