Y1 and +1 are used interchangeably to dial US phone numbers from a mobile (more on the plus sign).
When dialing from Israel:
Y is the International Exit Code and it is different for each Israel carrier: 00 (Kod Gisha), 012 (Smile Tikshoret), 013 (NetVision), 014 (Bezeq), 018 (Xfone)
Current time in Washington DC, United States: April 3 at 6:38 AM