Why this code places the - sign at the last - php

Question

I am using below code to display advanced custom field (date fields) in wordpress. I want two dates to appear like e.g 12 mar - 15 mar 2017.
I am using this code
<?php echo the_sub_field('start_date').' - '.the_sub_field('end_date'); ?>
How to do it please explain ?
Right now I am getting result like this 18/09/201722/09/2017 -
my full html code
<table style=" border-collapse: collapse; table-layout: fixed; width: 800px;">
<tbody>
<tr>
<td style="border: 1px solid #ddd;"><h5 style="color:#044970;">Refernce</h5></td>
<td style="border: 1px solid #ddd;"><h5 style="color:#044970;">Dates</h5></td>
<td style="border: 1px solid #ddd;"><h5 style="color:#044970;">Venue</h5></td>
<td style="border: 1px solid #ddd;"><h5 style="color:#044970;">Fees</h5></td>
<td style="border: 1px solid #ddd;"><h5 style="color:#044970;">CPE Credits</h5></td>
<td style="border: 1px solid #ddd;"><h5 style="color:#044970;">Regsitrations</h5></td>
</tr>
</tbody>
</table>
<table style=" border-collapse: collapse; table-layout: fixed; width: 800px;">
<tbody>
<?php
if( have_rows('sub_seminars') ):
while( have_rows('sub_seminars') ) : the_row();
?>
<tr>
<td style="border: 1px solid #ddd;"><h6><?php the_sub_field('ref'); ?></h6></td>
<td style="border: 1px solid #ddd;"><h6><?php echo the_sub_field('start_date').' - '.the_sub_field('end_date'); ?></h6></td>
<td style="border: 1px solid #ddd;"><h6><?php the_sub_field('venue'); ?></h6></td>
<td style="border: 1px solid #ddd;"><h6><?php the_sub_field('fees'); ?></h6></td>
<td style="border: 1px solid #ddd;"><h6><?php the_sub_field('cpe_credits'); ?></h6></td>
<td style="border: 1px solid #ddd;"><a href="<?php home_url(); ?>/euromatechnew/seminar-registrations?seminartitle=<?php echo htmlentities(urlencode(get_the_title())) ?>&seminarvenue=<?php echo the_sub_field('venue');
?>&seminardates=<?php echo the_sub_field('start_date').the_sub_field('end_date'); ?>&seminarref=<?php echo the_sub_field('ref') ?>" class="dt-btn" style="margin: 3px 0px 0px 0px">Register Now</a></td>
</tr>
<?php
endwhile;
endif;
?>
</tbody>
</table>

Answer 1

the_sub_field function already echoes data.
To get data and then echo it - use get_sub_field.
Proper code is:
<?php echo get_sub_field('start_date').' - '.get_sub_field('end_date'); ?>
Or as with your markup:
<td style="border: 1px solid #ddd;"><h6><?php the_sub_field('start_date');?> - <?php the_sub_field('end_date');?></h6></td>
To format your dates as you need you can use date fomatting functions of php, e.g date.