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 vs. 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 seconds while performing a 2-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.