In the Chart Supplement there is occasionally a
‡ symbol after the operating hours for control towers (and other time information) like what's shown below.
What does the ‡ symbol represent?
The ‡ symbol in the A/FD specifies that the location observes daylight savings time.
This is from the A/FD legend, which can be found online by going to the online digital A/FD and searching for an airport. The A/FD Legend link will then be at the top of the search results (unfortunately the link changes every cycle so I can't include an exact link).
6 TIME CONVERSION
Hours of operation of all facilities are expressed in Coordinated Universal Time (UTC) and shown as ‘‘Z’’ time. The directory indicates the number of hours to be subtracted from UTC to obtain local standard time and local daylight saving time UTC–5(–4DT). The symbol ‡ indicates that during periods of Daylight Saving Time effective hours will be one hour earlier than shown. In those areas where daylight saving time is not observed the (–4DT) and ‡ will not be shown. Daylight saving time is in effect from 0200 local time the second Sunday in March to 0200 local time the first Sunday in November. Canada and all U.S. Conterminous States observe daylight saving time except Arizona and Puerto Rico, and the Virgin Islands. If the state observes daylight saving time and the operating times are other than daylight saving times, the operating hours will include the dates, times and no ‡ symbol will be shown, i.e., April 15–Aug 31 0630–1700Z, Sep 1–Apr 14 0600–1700Z.
Or, alternatively, the answer is that the tower has the same (local) operating hours no matter if it's DST or not, i.e., it's always 9-5 local. However, since all hours are written in Zulu and the conversion to Zulu and local can change depending on if it's DST or not, this provides a quick shortcut to duplicating every entry's times.
More than anything else, this is a cautionary note that one should never speak 'x Standard time' unless you mean it - just say 'x time' instead.