Fixed in upstream commit 4dcbcd8. Turns out this feature never worked. Here's a simple program to exercise the functionality. You will need to adjust the timestamp to match what's in your log.

#include <stdio.h>
#include <stdlib.h>
#include <auparse.h>

#define LOG "./audit.log"

int main(void)
{
        int rc;
        char *error = NULL;
        const char *start_time = "ts:1208449363.116:3499";
        char expr[1024];
        auparse_state_t *au;

        au = auparse_init(AUSOURCE_FILE, LOG);
        if (au == NULL) {
                printf("master error\n");
                return 1;
        }
        snprintf(expr, sizeof(expr),
                "(\\timestamp_ex >= %s)",
                start_time);
        if (ausearch_add_expression(au, expr, &error, AUSEARCH_RULE_CLEAR)) {
                fprintf(stderr, "Criteria error: %s\n", error);
                free(error);
                return 1;
        }
        printf("Expression is OK\n");

        rc = ausearch_next_event(au);
        if (rc > 0)
                printf("Search is good\n");
        if (rc == 0)
                printf("Nothing found\n");
        if (rc < 0)
                printf("Error searching\n");

        auparse_destroy(au);
        return rc <= 0;
}

audit-2.8.4-1.el7 was built to address this issue.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:3237