ACS712 | Arduino library for ACS Current Sensor

To answer your first question about the unsigned long, with numerical data types the most-significant bit is usually used to determine if a number is positive or negative. However, this is at the cost of halving the positive capacity of the data type. If a negative value is not an expected outcome, then you can declare a data type as unsigned to use the most significant bit for a greater positive capacity.

So for example:

• An unsigned long using 4 bytes has a value range from 0 to 4,294,967,295
• A (signed) long using 4 bytes has a value range from -2,147,483,648 to 2,147,483,647

The use of UL at the end of a number explicitly tells the compiler that this value is an unsigned long. Note that the size of a long (or any other numerical types) may not be consistent across compilers.

See also - What's the difference between unsigned long/long/int in c/c++?

As for your other question, the Arduino docs describe the micros() function:

Returns the number of microseconds since the Arduino board began running the current program. This number will overflow (go back to zero), after approximately 70 minutes. On 16 MHz Arduino boards (e.g. Duemilanove and Nano), this function has a resolution of four microseconds (i.e. the value returned is always a multiple of four). On 8 MHz Arduino boards (e.g. the LilyPad), this function has a resolution of eight microseconds.

Note: there are 1,000 microseconds in a millisecond and 1,000,000 microseconds in a second.