A non-numeric value encountered in /wp-includes/functions.php on line 68

We recently updated our shared hosting to use PHP 7.3 (problem still existed in PHP 7.2 as well) – website is throwing the error in the title.

the function in question is:

function current_time( $type, $gmt = 0 ) {
switch ( $type ) {
    case 'mysql':
        return ( $gmt ) ? gmdate( 'Y-m-d H:i:s' ) : gmdate( 'Y-m-d H:i:s', ( time() + ( get_option( 'gmt_offset' ) * HOUR_IN_SECONDS ) ) );
    case 'timestamp':
        return ( $gmt ) ? time() : time() + ( get_option( 'gmt_offset' ) * HOUR_IN_SECONDS );
    default:
        return ( $gmt ) ? gmdate( $type ) : gmdate( $type, time() + ( get_option( 'gmt_offset' ) * HOUR_IN_SECONDS ) );
}}

Line 68 is:

return ( $gmt ) ? gmdate( 'Y-m-d H:i:s' ) : gmdate( 'Y-m-d H:i:s', ( time() + ( get_option( 'gmt_offset' ) * HOUR_IN_SECONDS ) ) );

The functions file has not been modified – any ideas what is causing this?