Alberto,

Andy is right. Radio clocks keep the receiver turned off most of
the time to save battery power. For example, my own alarm clock
("EuroChron") synchronises to DCF77 once per hour.
The "radio icon" in the LCD display starts flashing for a minute
between xx.59.00 and the full hour. If it fails to decode correctly
(parity check etc), the receiver (and flashing) stays on until valid
data is received.

BTW, my wife took the clock on a trip to Finland a few years ago
and accidentally pressed the reset button. Finland is too far away from
the DCF77 transmitter and, needless to say, there was no buttons
for manually setting the clock :-)

73
Johan SM6LKM