Time-in-grade(TIG) is a restriction on rapid advancement requiring General Schedule (GS) employees in the competitive service to complete a specific period of time at a GS grade level before becoming eligible for promotion to the next higher grade level. Candidates up to GS-05 may be advanced to no more than two GS grades above the lowest grade the employee held within the preceding 52 weeks under his or her latest non-temporary competitive appointment. Candidates at grade levels above the GS-05 must complete a minimum 52 weeks in that grade in order to advance to the next higher grade in the appropriate line of progression. TIG is one of two criteria needed for promotion, and is not to be confused with the requirement for qualifying specialized experience for a position.
The slides in the attached link provide the process to credit non-GS or NSPS experience in accordance with 5 CFR 300.605(b),
Time-In-Grade (powerpoint slides).