合う and 合わせる: a fit and an adjustment to a standard
Compare a fit in size, time, or taste with adjustment, and distinguish に marking a suitable target from に marking an adjustment standard.
A fit in size
この靴は足に合いません。 says that these shoes do not fit the speaker's feet. 合う, read あう, concerns suitability; the sentence does not specify whether they are too large or too small.
長く歩くので、足に合う靴を選びます。 means that the speaker will choose well-fitting shoes for a long walk. The focus on fit differs from the visual suitability of 似合う.
Adjust to a standard
時計を正しい時刻に合わせました。 means that someone set a clock to the correct time. 合わせる, read あわせる, marks the adjusted object with を and the standard with に.
音楽に合わせて手をたたいてください。 asks someone to clap in time with the music. に gives the timing standard, while 手を belongs to the clapping action.
Agreement between people
友達とは食べ物の好みが合います。 says that the speaker and a friend share compatible tastes in food. It concerns preferences rather than physical dimensions.
予定が合わないので、別の日を探しましょう。 proposes finding another day because the schedules do not match. It describes a mismatch, not an adjustment already made.
What are the adjusted object and the standard in 時計を正しい時刻に合わせる?
Show the answer and why
The clock, 時計を, is adjusted; the correct time, 正しい時刻に, is the standard. It does not mean changing the correct time itself.