Vaisala HM40 - Manual (Page 20)

19
When the screen is held, the top of
the display displays the current
date and time. This is useful when
you want to record the current data
(take a photograph or write it
down).
If time has not been set, the top of
the screen will simply read “HOLD”.
When the view is held, you can reset all
measurement data by pressing the Reset button.
This will clear all graphs and tagged points.
If you keep pressing the right function button
(long press), the meter will tag the point without
freezing the screen.