Previous studies report that monitoring the passing of time by checking a clock either frequently or strategically (immediately before a target time) improves the likelihood of remembering to perform a planned intention at a specific time (i.e., time-based prospective memory [TBPM]). Critically, strategicness of clock-checking is usually measured as the number of clock-checks during the last time interval before the target time-an operationalization where strategicness actually intertwines with absolute frequency of clock-checking and may not properly account for age effects in TBPM performance. To disentangle the respective contribution of frequent versus strategic clock-checking to the age-related decrease in TBPM performance, we propose a new, more fine-grained indicator of strategicness (i.e., relative clock-checking), which accounts for interindividual differences in the total frequency of clock-checking (i.e., absolute clock-checking). In this study, 223 participants from an adult lifespan sample (age range = 19-86, M = 45.61, SD = 17.24; 70% women) had to remember to push the ENTER key every 60 s while performing a two-back picture decision task. Together, relative and absolute clock-checking fully mediated the negative age effect on TBPM and explained 53.6% of the variance of TBPM performance. Complementary analyses revealed that both indicators were needed to fully mediate the effect of age on TBPM, but that strategic (i.e., relative) clock-checking was a stronger predictor of TBPM performance than absolute clock-checking. These results stress the importance of considering both aspects of clock-checking to investigate time monitoring in laboratory TBPM tasks and age effects therein, and provide avenues of intervention for improving older adults' TBPM. (PsycInfo Database Record (c) 2022 APA, all rights reserved).