I know that I can use key frames in various fields of the Geometry Nodes. But I want a more explicit reference of current frame number. The only thing I found is the 'Frame' layout to enclose nodes.
Is there a 'Frame' node which outputs the current frame number?
I don't think there will be a node specifically designed to access the frame number. Although, like in the shader editor where this information is also sometimes needed, you can use a very simple driver to access the frame number directly.
Say you have this kind of setup and would like the Value node to output the frame number.
Left-Click in the value field like you would do if you wanted to input a new value, and type #frame. This will automatically create a driver (which can get a bit complicated if you want to go into the details, but in this case there is no need). This driver will have the same value as the current frame.
The field becomes purple to indicate it's now driven. To remove the driver, right click in the field and choose : Delete Driver.
Say you would want the value to increase more slowly. There are two methods to do it : Either tweak the driver (lesser solution) or add a math node afterwards (preferred solution, at least if you aren't really at ease with drivers).
Method 1 : You can add an operation after the frame driver.
Method 2 (better) : Use a math node. this gives you control over the multiplier and you can link it to the input node to directly tweak it into the modifier properties editor.
Tip : Use F2 or Right Click > Rename to rename the node to Frame.
frame_end property which is attributed to the current scene
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More modern answer (>= Blender 3.1; Released March 9th, 2022)
Blender 3.1 added the "Scene Time" node. It offers the current scene time in seconds and in frames (both as float values).
Link to the corresponding documentation: https://docs.blender.org/manual/en/latest/compositing/types/input/scene_time.html
For frame_end as mentioned here, there isn't a global variable for it. But most properties like this can be called in drivers using "Input Variables".
Making them manually is a bit slow, the fastest way is to 
RMB the property you need > Copy As New Driver, then make a Value node, RMB > Paste Driver.
You can use that node as is. But if you want to have that Input Variable in another driver, I.E. for use in a scripted expression, you can RMB > Edit Driver, and from there you have buttons to copy/paste input variables.
I.E. If you want a driver to give frame_end - frame, you can do this:
