Early grand unification theories (GUTs) such as the Georgi–Glashow model, which were the first consistent theories to suggest proton decay, postulated that the proton's half-life would be at least 10 years. As further experiments and calculations were performed in the 1990s, it became clear that the proton half-life could not lie below 10 years. Many books from that period refer to this figure for the possible decay time for baryonic matter. More recent findings have pushed the minimum proton half-life to at least 10 –10 years, ruling out the simpler GUTs (including minimal SU(5) / Georgi–Glashow) and most non-SUSY models. The maximum upper limit on proton lifetime (if unstable), is calculated at 6 × 10 years, a bound applicable to SUSY models, with a maximum for (minimal) non-SUSY GUTs at 1.4 × 10 years.