Añadiendo unos márgenes negativos corregirás la separación de "button"
Código css:
Ver original.prev {margin-right:-5px;}
.today {margin: 0 -5px;}
.next {margin-left: -5px;}
ajusta los valores para que sean exactos.
Sin entrar en el error que es colocar de esa forma el elemento "button".
Cita: document type does not allow element "button" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag
Cita: <button class="prev"><img src="izq.jpg" alt="image" /></button>
The mentioned element is not allowed to appear in the context in which you've placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.
Si no fuese realmente un botón y sí un menú, sería mejor que hicieses eso con una lista
Un saludo