Piano music often has multiple parts (or lines) written on each stave. For historical reasons (from choral music) these are usually known as voices. Although it is possible to have more than two voices per stave, there are usually not more than two. To differentiate between notes written in two voices on one stave, one voice has stems pointing upwards, the other has stems pointing downwards, regardless of how high or low the notes are.
In the short extract from Messiaen you posted, the right hand part (upper stave) has two voices. So the rhythms for these notes are read at the same time not necessarily in the order that they appear from left to right. However, this is slightly confusing in the extract you show, as really there should be a rest at the beginning of the bar in the right-hand lower voice; instead only the two crotchet notes are shown. Below is the extract you posted notated correctly:
It's worth noting that the left-hand part also has two voices, but as their rhythms are not independent, the rhythm is much easier to work out!