Source

File: wp-includes/customize/class-wp-customize-date-time-control.php

public function format_gmt_offset( $offset ) {
		if ( 0 <= $offset ) {
			$formatted_offset = '+' . (string) $offset;
		} else {
			$formatted_offset = (string) $offset;
		}
		$formatted_offset = str_replace(
			array( '.25', '.5', '.75' ),
			array( ':15', ':30', ':45' ),
			$formatted_offset
		);
		return $formatted_offset;
	}