What does the clock-like symbols under plastic containers mean?
This is the date the item was manufactured. The upper dial gives the year, the middle one, the month.

The bottom indicator could be a mold identifier, and in some cases, it might be a shift indicator (although this would typically be a clock going from 1 to 3, 4, or 6 depending on how shifts are allocated rather than the example showing 0 to 5).

See e.g. this manufacturer's datasheet on mold indicators: http://www.plastixs.com/pdfs/datecodeti.pdf