Would the VWP clock reset if I travel to Europe from the US?

You should be fine. When you return to the US, if you are admitted, you should be granted a new 90-day period of admission.

The visa waiver program has some rules that are designed to prevent people from nipping over the border to Canada, Mexico, or an "adjacent island" in order to gain a new 90-day period of admission to the United States. The restriction is articulated at 8 CFR 217.3(b):

(b) Readmission after departure to contiguous territory or adjacent island.

An alien admitted to the United States under this part may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Pilot Program admission period if he or she is otherwise admissible and meets all the conditions of this part with the exception of arrival on a signatory carrier.

Foreign contiguous territory means Canada and Mexico; adjacent islands is non-exclusively defined at 8 USC 1101(b)(5) with a list comprising mostly Caribbean and Atlantic islands:

(5) The term “adjacent islands” includes Saint Pierre, Miquelon, Cuba, the Dominican Republic, Haiti, Bermuda, the Bahamas, Barbados, Jamaica, the Windward and Leeward Islands, Trinidad, Martinique, and other British, French, and Netherlands territory or possessions in or bordering on the Caribbean Sea.

This provision does not apply to a traveler who has departed to Europe. You are therefore likely to receive a new 90-day period of admission when you return to the United States.

Do I count the day I'm leaving and the day I'm arriving?

As outlined above, it does not matter in your case, but there seem to be pictures of US admission stamps available on the internet that suggest both methods of counting. It is safer to plan to leave the US on the 90th day counting the day of arrival.