PHP, 102 95 90 bytes

Saved 7 bytes thanks to @TheLethalCoder & by not factoring

Saved another 5 bytes by concatenating

<? $b=1553990400-time();echo floor($b/$d=86400).date(':H:i:s', $b%$d);header('Refresh:1');

This is my first golf, so I'm probably missing quite a few tricks, but here you are regardless.

As for @chocochaos' PHP answer that would otherwise trump this, I believe it is flawed for reasons I have explained in my comments, but as I'm new here I might be wrong.

PHP, 102 95 90 bytes

Saved 7 bytes thanks to @TheLethalCoder & by not factoring

Saved another 5 bytes by concatenating

<? $b=1553990400-time();echo floor($b/$d=86400).date(':H:i:s', $b%$d);header('Refresh:1');

This is my first golf, so I'm probably missing quite a few tricks, but here you are regardless.

As for @chocochaos' PHP answer that would otherwise trump this, I believe it is flawed for reasons I have explained in my comments, but as I'm new here I might be wrong. Or I'm just a newbie :)

PHP, 102 95 bytes

Saved 7 bytes thanks to @TheLethalCoder & by not factoring

<? $b=1553990400-time();echo floor($b/86400);echo date(':H:i:s', $b%86400);header('Refresh:1');

This is my first golf, so I'm probably missing quite a few tricks, but here you are regardless.

As for @chocochaos' PHP answer that would otherwise trump this, I believe it is flawed for reasons I have explained in my comments, but as I'm new here I might be wrong.

PHP, 102 95 90 bytes

Saved 7 bytes thanks to @TheLethalCoder & by not factoring

Saved another 5 bytes by concatenating

<? $b=1553990400-time();echo floor($b/$d=86400).date(':H:i:s', $b%$d);header('Refresh:1');

This is my first golf, so I'm probably missing quite a few tricks, but here you are regardless.

As for @chocochaos' PHP answer that would otherwise trump this, I believe it is flawed for reasons I have explained in my comments, but as I'm new here I might be wrong.

PHP, 102 95 bytes

Saved 7 bytes thanks to @TheLethalCoder & by not factoring

<? $b=1553990400-time();echo floor($b/86400);echo date(':H:i:s', $b%86400);header('Refresh:1');

This is my first golf, so I'm probably missing quite a few tricks, but here you are regardless.

As for @chocochaos' PHP answer that would otherwise trump this, I believe it is flawed for reasons I have explained in my comments, but as I'm new here I might be wrong.

PHP, 102 95 bytes

Saved 7 bytes thanks to @TheLethalCoder & by not factoring

<? $b=1553990400-time();echo floor($b/86400);echo date(':H:i:s', $b%86400);header('Refresh:1');

This is my first golf, so I'm probably missing quite a few tricks, but here you are regardless.

As for @chocochaos' PHP answer that would otherwise trump this, I believe it is flawed for reasons I have explained in my comments, but as I'm new here I might be wrong.