You are being hit by auto-correction while entering a 3/4
fraction (and no, it shouldn’t happen with 15/4
). You can Ctrl+Z once immediately after such correction happened. Or you disable the auto-correction entirely under Tools → AutoCorrect Options…, tab Options, uncheck Use replacement table. Or delete just the 1/2
, 1/4
and 3/4
replacements from the list in the Replace tab (enter 1
in the Replace edit field to jump to the entry). Note however that AutoCorrect Options apply to all LibreOffice modules, so if you want to continue to use these replacements in Writer rather switch Use replacement table on/off.
Also make sure the Date Acceptance Patterns are set correctly, what JohnSUN said, but as you seem to use the English (UK) locale, the patterns for you would be D/M/Y;D/M
instead of the English (USA) M/D/Y;M/D