I think you need a switch boolean.
1)Create a Global Number Variable called Switch with a value of 0. And also Create your Global Number Variable called Timer with a value of 60.
2) Apply this event:
*Timer <= 0
----> Set Global Number Variable (Switch) to 1.
*Switch = 0
----->Subtract from Timer (dt)
*Switch = 1
------>Add from Timer (dt)
3) Tip: It's a good practice to add this event for accuracy:
*Timer < 0
-----> Set Timer to 0