頃 / ごろ【ころ / ごろ】

頃 marks a time. Straight after a clock time or a date it is read ごろ and means "around"; after の, an adjective or a verb it is read ころ and means "the time when", "back when".

ころ: the time when

ころ names a stretch of time rather than a moment – a time of your life, a period back then. A noun takes の before it, a な-adjective takes な, and a verb is in the plain form: the た-form looks back, the dictionary form is "around when it is about to happen".

"At that time" is そのころ, and "back then", looking back with some feeling, is あのころ.

ごろ: around a time

ごろ goes straight after a clock time, a day, a month or a year, with no の. に after it is optional.

いまごろ今ごろ means "about now" or "by now", often in a guess about what someone else is doing.

ごろ or ぐらい

ごろ marks only a point in time. For a length of time, the word is ぐらい.

• × 3時間ごろ待ちました → 3時間ぐらい待ちました

In conversation ぐらい also goes on a clock time – さんじ3時ぐらいにあおう会おう – so the mix-up only goes one way.