Cassie's Watch is an object owned by Cassandra Railly. She is seen wearing it constantly. When she ultimately succumbs to the plague, she is still wearing the watch. It is found by Cole in 2043 and brought back to Cassie in 2013 to prove to her that he is a time traveler.
Cole splinters in and finds Cassandra in Baltimore. He shows her the 2043 iteration of the watch. As she watches the timepiece, Cole scratches the crystal of the 2013 iteration. The scratch appears on the 2043 iteration.
After Cole and Cassie are taken captive by security officers of Markridge Group, Leland Goines refers to a paradox. Cole offers to show him a paradox. Cole places the two iterations of the watch in contact and a massive explosion ensues. After their escape, Cole hands Cassie the present-day version of her watch, explaining that it will save their life. Cassie continues to wear the watch for many years.
A dying Cassie is still wearing the watch when she meets Cole at the Baltimore CDC. After she dies and Cole splinters away to 2043, the scratch that Cole had made in 2013 on the crystal of the watch disappears, erased by Time, so that it can be found unscratched in 2043 and complete the cycle.