Computer Vision - Algorithm for reconstructing tilted image
It detects 4 markers lying on the corners of the note. One of the markers is unique to able to determine the orientation. Because the actual size and layout of the form is known, a perspective warp can be computed, bringing the scanned image to a canonical (fronto-parallel) form. Then, OCR is carried out in the region where date and time is expected. The rest of the content are taken as image and saved on the smartphone.

For OCR, character segmentation and classification might be your key words.
For Marker Detection, generally black-white polygonal regions are detected and expected to be quad-formed. The rest of the blobs are filtered out and you end up with the markers.