If there is an undesired auto-correct, hit Ctrl+Z immediately. In some cases you may have to hit Ctrl+Z twice (e.g., typing 1/2/ will immediately change 1/2 to ½ so the cell will show ½/, first Ctrl+Z removes the /, the second Ctrl+Z will change ½ back to 1/2, and then you can hit / to get 1/2/ and you can then type in the year).
To stop auto-correct on 1/4, 1/2, and 3/4 only, but have corrections for other key combos and common transpositions, go into Tools, AutoCorrect Options, “Replace” tab, move highlight bar to 1/2, and hit the Delete button, and repeat for 1/4 and 3/4.
Or you can stop all auto-corrections by going into Tools, AutoCorrect Options, “Options” tab, and uncheck “Use replacement table”.
Without auto-correction or undoing an auto-correction with Ctrl+z, the 1/2 will be interpreted as a date (at least with the US locality).
To get 1/2 to come up as 0.5, you can type =1/2 followed by a tab, space, or Enter.
The “=” tells Calc that what you type next is part of a formula, and the formula will be put into the cell (so going back to the cell will show =1/2 in the input line).