add getUnixTime function

[howard-wong]

No description provided.

[sleemanj]

Thank you for the PR.

Leap year calculation appears incomplete, for example 2100 will not be a leap year but your code would include it I think.

"dim" and "a_dim" are not very descriptive names

I'm not sure about dim[i] unless this is a new feature of GCC handling it automatically, since a_dim is in PROGMEM the program memory reading functions (pgm_read_byte specifically) need to be used, without retrieving the data in this manner you are just grabbing random junk from a ram location, not the flash location.