Prepositions like AT, ON and IN are used in English to say where things exist in space or time. Our brains try to organise space into three dimensions, and we use a similar system to describe time. One-dimensional points are moments in time and English usually fixes both of these with AT. ON is used when we see two-dimensional surfaces or lines, while IN is reserved for bigger, three-dimensional spaces that things can enter or be enclosed by.
AT for Moments
Atomic AT refers to single, indivisible points. It wants to be millimetrically precise, but sometimes has to accept approximations like “AT the top of this hill” when we don’t give it enough data. With time, it is usually very specific: AT midnight, AT 14.35 or AT this moment, but again it sometimes has to compromise with things like AT lunchtime or AT the end of the working day. This might also explain the surprising English tendency to use AT for holiday times, too, so we do things AT the weekend, AT Christmas or AT Easter.
ON for Days
ON is a bit like an indecisive version of IN that touches the surface (or floats above it) without really getting involved. You probably like to sleep IN your bed at night while a claustrophobic cat will definitely sleep ON it (leaving hairs ON or IN the blanket, depending on how deeply the hairs get into the fabric…). With time, ON is clear: it’s for days, however you choose to describe them. Your event can take place ON Wednesday, ON 22 June 2026, or ON the last working day of each month, but if want to fix a particular day, you need ON.
IN for Bigger Periods
That leaves you with IN for bigger, more approximate times. You remember that you started a project IN 2022, maybe you also remember that it was IN July, or at least, IN the summer, but you’ve probably forgotten the specific day you started ON… It’s equally good for decades (I was born IN the 70s), centuries (we’re currently IN the 21st) and other unimaginably big things like IN ages or IN your life.
IN is where time prepositions in English start to lose their precision and refer to more open periods or durations. My next blog will look at the reasons why BY is often used for deadlines, even when the deadline falls ON a day, and how to differentiate FROM and SINCE when you want to set a starting point.
