strftime:
    %a – The abbreviated weekday name (``Sun’‘)
    %A – The full weekday name (``Sunday’‘)
    %b – The abbreviated month name (``Jan’‘)
    %B – The full month name (``January’‘)
    %c – The preferred local date and time representation
    %d – Day of the month (01..31)
    %H – Hour of the day, 24-hour clock (00..23)
    %I – Hour of the day, 12-hour clock (01..12)
    %j – Day of the year (001..366)
    %m – Month of the year (01..12)
    %M – Minute of the hour (00..59)
    %p – Meridian indicator (``AM’’ or ``PM’‘)
    %S – Second of the minute (00..60)
    %U – Week number of the current year,
        starting with the first Sunday as the first
        day of the first week (00..53)
    %W – Week number of the current year,
        starting with the first Monday as the first
        day of the first week (00..53)
    %w – Day of the week (Sunday is 0, 0..6)
    %x – Preferred representation for the date alone, no time
    %X – Preferred representation for the time alone, no date
    %y – Year without a century (00..99)
    %Y – Year with century
    %Z – Time zone name
    % – Literal ``’’ character

Source: the cheat gem (relevant blog post)

Cheat has mostly gone offline, but I still refer to this cheat sheet all the time, years and years later. Perhaps that means that strftime has terrible syntax, but that doesn’t stop it from being ubiquitous.